diff --git a/asl_rulebook2/bin/fixup_mmp_pdf.py b/asl_rulebook2/bin/fixup_mmp_pdf.py
index 867574f..24f5250 100755
--- a/asl_rulebook2/bin/fixup_mmp_pdf.py
+++ b/asl_rulebook2/bin/fixup_mmp_pdf.py
@@ -2,6 +2,8 @@
 """ Fixup issues in the MMP eASLRB. """
 
 import os
+import threading
+import time
 
 from pikepdf import Pdf, Page, OutlineItem, Encryption, make_page_destination
 import click
@@ -10,7 +12,7 @@ from asl_rulebook2.utils import log_msg_stderr
 
 # ---------------------------------------------------------------------
 
-def fixup_mmp_pdf( fname, output_fname, fix_zoom, optimize_web, rotate, log=None ):
+def fixup_mmp_pdf( fname, output_fname, fix_zoom, optimize_web, rotate, log=None, relinq=None ):
     """Fixup the MMP eASLRB PDF."""
 
     # NOTE: v1.03 had problems with links within the PDF being of type /Fit rather than /XYZ,
@@ -91,15 +93,28 @@ def fixup_mmp_pdf( fname, output_fname, fix_zoom, optimize_web, rotate, log=None
 
         # save the updated PDF
         log_msg( "progress", "Saving the fixed-up PDF..." )
-        # NOTE: Setting a blank password will encrypt the file, but doesn't require the user to enter a password
-        # when opening the file (but it will be marked as "SECURE" in the UI).
+        # NOTE: Setting a blank password will encrypt the file, but doesn't require the user
+        # to enter a password when opening the file (but it will be marked as "SECURE" in the UI).
         enc = Encryption( owner="", user="" )
-        def save_progress( pct ):
-            if pct > 0 and pct % 10 == 0:
-                log_msg( "verbose", "- Saved {}%.", pct )
-        pdf.save( output_fname, encryption=enc, linearize=optimize_web,
-            progress = save_progress
+        # NOTE: We can't log progress messages if we're being run from the webapp, since log_msg()
+        # will try to relinquish the CPU, but it will be in the wrong thread. We could disable this,
+        # but it's more trouble than it's worth.
+        thread = SavePdfThread( pdf,
+            output_fname, enc, optimize_web,
+            log_msg = None if relinq else log_msg
         )
+        thread.start()
+        pass_no = 0
+        while True:
+            if thread.done:
+                break
+            pass_no += 1
+            if relinq:
+                relinq( "Saving PDF: {}".format( pass_no ), delay=1 )
+            else:
+                time.sleep( 1 )
+        if thread.exc:
+            raise thread.exc
 
         # compare the file sizes
         old_size = os.path.getsize( fname )
@@ -112,6 +127,40 @@ def fixup_mmp_pdf( fname, output_fname, fix_zoom, optimize_web, rotate, log=None
                 abs(ratio), "larger" if ratio > 0 else "smaller"
             )
 
+# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+class SavePdfThread( threading.Thread ):
+    """Save the PDF in a background thread."""
+
+    def __init__( self, pdf, fname, enc, optimize_web, log_msg ):
+        # initialize
+        super().__init__( daemon=True )
+        self.pdf = pdf
+        self.fname = fname
+        self.enc = enc
+        self.optimize_web = optimize_web
+        self._log_msg = log_msg
+        # initialize
+        self.done = False
+        self.exc = None
+
+    def run( self ):
+        """Run the worker thread."""
+        try:
+            self.pdf.save( self.fname,
+                encryption=self.enc, linearize=self.optimize_web,
+                progress=self._log_progress
+            )
+        except Exception as ex: #pylint: disable=broad-except
+            self.exc = ex
+        finally:
+            self.done = True
+
+    def _log_progress( self, pct ):
+        """Log progress."""
+        if self._log_msg and pct > 0 and pct % 10 == 0:
+            self._log_msg( "verbose", "- Saved {}%.", pct )
+
 # ---------------------------------------------------------------------
 
 @click.command()
diff --git a/asl_rulebook2/bin/prepare_pdf.py b/asl_rulebook2/bin/prepare_pdf.py
index 965f9ee..b90617c 100755
--- a/asl_rulebook2/bin/prepare_pdf.py
+++ b/asl_rulebook2/bin/prepare_pdf.py
@@ -23,7 +23,12 @@ _COMPRESSION_CHOICES = [
 
 # ---------------------------------------------------------------------
 
-def prepare_pdf( pdf_file, title, targets_fname, vo_notes_fname, yoffset, output_fname, compression, gs_path, log_msg ):
+def prepare_pdf( pdf_file,
+    title, targets_fname, vo_notes_fname, yoffset,
+    output_fname, compression,
+    gs_path,
+    log_msg, relinq=None
+):
     """Prepare the MMP eASLRB PDF."""
 
     # load the targets
@@ -47,7 +52,7 @@ def prepare_pdf( pdf_file, title, targets_fname, vo_notes_fname, yoffset, output
                 pdf_file
             ]
             start_time = time.time()
-            subprocess.run( args, check=True )
+            _run_subprocess( args, "compression", relinq )
             elapsed_time = time.time() - start_time
             log_msg( "timestamp", "- Elapsed time: {}".format(
                 datetime.timedelta( seconds=int(elapsed_time) ) )
@@ -101,7 +106,7 @@ def prepare_pdf( pdf_file, title, targets_fname, vo_notes_fname, yoffset, output
         args.extend( [ "-f", pdf_file ] )
         args.append( pdfmarks_file.name )
         start_time = time.time()
-        subprocess.run( args, check=True )
+        _run_subprocess( args, "pdfmarks", relinq )
         elapsed_time = time.time() - start_time
         log_msg( "timestamp", "- Elapsed time: {}".format(
             datetime.timedelta( seconds=int(elapsed_time) ) )
@@ -109,6 +114,33 @@ def prepare_pdf( pdf_file, title, targets_fname, vo_notes_fname, yoffset, output
 
 # ---------------------------------------------------------------------
 
+def _run_subprocess( args, caption, relinq ):
+    """Run an external process."""
+    proc = subprocess.Popen( args )
+    try:
+        pass_no = 0
+        while True:
+            pass_no += 1
+            # check if the external process has finished
+            rc = proc.poll()
+            if rc is not None:
+                # yup - check its exit code
+                if rc != 0:
+                    raise RuntimeError( "Sub-process \"{}\" failed: rc={}".format( caption, rc ) )
+                break
+            # delay for a bit before checking again
+            if relinq:
+                relinq( "Waiting for {}: {}".format( caption, pass_no ), delay=1 )
+            else:
+                time.sleep( 1 )
+    except ( Exception, KeyboardInterrupt ):
+        # NOTE: We want to kill the child process if something goes wrong, and while it's not
+        # 100%-guaranteed that we will get here (e.g. if we get killed), it's good enuf.
+        proc.terminate()
+        raise
+
+# ---------------------------------------------------------------------
+
 @click.command()
 @click.argument( "pdf_file", nargs=1, type=click.Path(exists=True,dir_okay=False) )
 @click.option( "--title", help="Document title." )
diff --git a/asl_rulebook2/webapp/__init__.py b/asl_rulebook2/webapp/__init__.py
index 102946f..53b081f 100644
--- a/asl_rulebook2/webapp/__init__.py
+++ b/asl_rulebook2/webapp/__init__.py
@@ -50,9 +50,6 @@ def _on_sigint( signum, stack ): #pylint: disable=unused-argument
 
 # ---------------------------------------------------------------------
 
-# disable the Flask startup banner
-flask.cli.show_server_banner = lambda *args: None
-
 # initialize Flask
 app = Flask( __name__ )
 
diff --git a/asl_rulebook2/webapp/prepare.py b/asl_rulebook2/webapp/prepare.py
index b8a8d2c..c5b61d7 100644
--- a/asl_rulebook2/webapp/prepare.py
+++ b/asl_rulebook2/webapp/prepare.py
@@ -1,9 +1,7 @@
 """ Analyze the MMP eASLRB PDF and prepare the data files. """
 
-import threading
 import zipfile
 import io
-import time
 import base64
 import traceback
 import logging
@@ -33,21 +31,20 @@ def prepare_data_files():
     download_url = url_for( "download_prepared_data" )
 
     # initialize the socketio server
+    # NOTE: We wait until the client tells us to start processing (instead of when the POST data arrives),
+    # since it might not be ready to receive events, and miss the first few.
     sio = globvars.socketio_server
-    if not sio:
-        raise RuntimeError( "The socketio server has not been started." )
     @sio.on( "start" )
-    def on_start( data ): #pylint: disable=unused-variable,unused-argument
-        # start the worker thread that prepares the data files
-        # NOTE: We don't do this when the POST request comes in, but wait until the client
-        # tells us it's ready (otherwise, it might miss the first event or two).
-        def worker():
-            try:
-                _do_prepare_data_files( args, download_url )
-            except Exception as ex: #pylint: disable=broad-except
-                _logger.error( "PREPARE ERROR: %s\n%s", ex, traceback.format_exc() )
-                globvars.socketio_server.emit( "error", str(ex) )
-        threading.Thread( target=worker, daemon=True ).start()
+    def on_start(): #pylint: disable=unused-variable
+        # NOTE: We used to do this in a background thread (when we were using the Flask development server),
+        # but flask-socketio + eventlet handles concurrency differently, and we now do it synchronously,
+        # and periodically relinquish the CPU, so that we remain responsive (otherwise the client pings timeout,
+        # and it disconnects).
+        try:
+            _do_prepare_data_files( args, download_url )
+        except Exception as ex: #pylint: disable=broad-except
+            _logger.error( "PREPARE ERROR: %s\n%s", ex, traceback.format_exc() )
+            globvars.socketio_server.emit( "error", str(ex) )
 
     return "ok"
 
@@ -100,6 +97,11 @@ def _do_prepare_data_files( args, download_url ):
                 msg = msg[2:]
             sio.emit( msg_type, msg )
             msg_types.add( msg_type )
+            # NOTE: There's no particular significance in relinquishing the CPU here, but this function
+            # is called regularly during processing, so it's a convenient place to do it.
+            # This function also gets passed into the low-level extract code (as a logging handler),
+            # which results in that code also relinquishing at regular intervals.
+            _relinq( msg )
 
         # NOTE: The plan was to allow the user to change the default parameters in the UI,
         # but this can be done (ahem) later. For now, if they really need to change something,
@@ -143,7 +145,8 @@ def _do_prepare_data_files( args, download_url ):
                 targets_file.name, vo_notes_file.name, 5,
                 prepared_file.name, "ebook",
                 gs_path,
-                log_msg
+                log_msg,
+                relinq = _relinq
             )
 
         # fixup the PDF
@@ -153,7 +156,8 @@ def _do_prepare_data_files( args, download_url ):
             fixup_mmp_pdf( prepared_file.name,
                 fixedup_file.name,
                 False, True, True,
-                log_msg
+                log_msg,
+                relinq = _relinq
             )
             # read the final PDF data
             with open( fixedup_file.name, "rb" ) as fp:
@@ -179,6 +183,10 @@ def _do_prepare_data_files( args, download_url ):
     # NOTE: We don't bother shutting down the socketio server, since the user
     # has to restart the server, using the newly-prepared data files.
 
+def _relinq( msg=None, delay=0 ): #pylint: disable=unused-argument
+    """Relinquish the CPU (to keep the webapp server responsive)."""
+    globvars.socketio_server.sleep( delay )
+
 # ---------------------------------------------------------------------
 
 @app.route( "/prepare/download" )
@@ -215,5 +223,5 @@ def _test_progress( npasses=100, status=10, warnings=None, errors=None, delay=0.
             sio.emit( "error", "Progress {}: error".format( 1+i ) )
         else:
             sio.emit( "progress", "Progress {}.".format( 1+i ) )
-        time.sleep( float( delay ) )
+        _relinq( delay=float(delay) )
     sio.emit( "done" )
diff --git a/asl_rulebook2/webapp/run_server.py b/asl_rulebook2/webapp/run_server.py
index 7f9a977..03840d3 100755
--- a/asl_rulebook2/webapp/run_server.py
+++ b/asl_rulebook2/webapp/run_server.py
@@ -7,6 +7,7 @@ import urllib.request
 import time
 import glob
 
+import flask_socketio
 import click
 
 from asl_rulebook2.webapp import app, globvars
@@ -22,16 +23,16 @@ def main( bind_addr, data_dir, force_init_delay, flask_debug ):
     """Run the webapp server."""
 
     # initialize
-    port = None
+    flask_port = None
     if bind_addr:
         words = bind_addr.split( ":" )
-        host = words[0]
+        flask_host = words[0]
         if len(words) > 1:
-            port = words[1]
+            flask_port = words[1]
     else:
-        host = app.config.get( "FLASK_HOST", "localhost" )
-    if not port:
-        port = app.config.get( "FLASK_PORT_NO" )
+        flask_host = app.config.get( "FLASK_HOST", "localhost" )
+    if not flask_port:
+        flask_port = app.config.get( "FLASK_PORT_NO" )
     if not flask_debug:
         flask_debug = app.config.get( "FLASK_DEBUG", False )
 
@@ -42,9 +43,9 @@ def main( bind_addr, data_dir, force_init_delay, flask_debug ):
         app.config["DATA_DIR"] = data_dir
 
     # validate the configuration
-    if not host:
+    if not flask_host:
         raise RuntimeError( "The server host was not set." )
-    if not port:
+    if not flask_port:
         raise RuntimeError( "The server port was not set." )
 
     # monitor extra files for changes
@@ -75,36 +76,47 @@ def main( bind_addr, data_dir, force_init_delay, flask_debug ):
     if force_init_delay > 0:
         def _start_server():
             time.sleep( force_init_delay )
-            url = "http://{}:{}".format( host, port )
+            url = "http://{}:{}".format( flask_host, flask_port )
             _ = urllib.request.urlopen( url )
         threading.Thread( target=_start_server, daemon=True ).start()
 
-    # check if the user needs to prepare their data files
-    if not app.config.get( "DATA_DIR" ):
-        # yup - initialize the socketio server
-        init_prepare_socketio( app )
-
     # run the server
-    app.run( host=host, port=port, debug=flask_debug,
-        extra_files = extra_files
-    )
+    run_server( flask_host, flask_port, flask_debug, extra_files )
 
 # - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 
-def init_prepare_socketio( flask_app ):
-    """Initialize the socketio server needed to prepare the data files."""
-    # NOTE: We only set this up if it's needed (i.e. because there is no data directory,
-    # and the user needs to prepare their data files), rather than always having it running
-    # on the off-chance that the user might need it :-/
-    # NOTE: socketio doesn't really work well with threads, and it's tricky to get it to
-    # send events to the client if we're using e.g. eventlet:
-    #   https://stackoverflow.com/questions/43801884/how-to-run-python-socketio-in-thread
-    #   https://python-socketio.readthedocs.io/en/latest/server.html#standard-threads
-    # Using native threads is less-performant, but it's not an issue for us, and it works :-/
-    import socketio
-    sio = socketio.Server( async_mode="threading" )
-    flask_app.wsgi_app = socketio.WSGIApp( sio, flask_app.wsgi_app )
+def run_server( host, port, debug, extra_files=None ):
+    """Run the webapp server."""
+
+    # NOTE: flask-socketio + eventlet handles concurrency differently to the Flask development server,
+    # and we need to remain responsive, otherwise pings from the socketio client will timeout, and it will
+    # disconnect (and show a big warning in the UI that the server has gone away). To avoid this,
+    # we relinquish the CPU regularly, but just in case, we increase the ping timeout (and allow the user
+    # to increase it even further, if necessary). This should only be an issue when preparing the data files,
+    # since the main program doesn't use socketio.
+    # NOTE: Setting the timeout high shouldn't be a problem, since if the server really does go away,
+    # the connection will be dropped, and the front-end Javascript will detect that immediately.
+    ping_timeout = app.config.get( "SOCKETIO_PING_TIMEOUT", 30 )
+
+    # run the server
+    sio = flask_socketio.SocketIO( app,
+        async_mode = "eventlet",
+        ping_timeout = ping_timeout
+    )
     globvars.socketio_server = sio
+    args = {
+        "debug": debug,
+        "log_output": False
+    }
+    if extra_files:
+        args.update( {
+            "use_reloader": True,
+            "reloader_options": { "extra_files": extra_files },
+        } )
+    sio.run( app,
+        host=host, port=port,
+        **args
+    )
 
 # ---------------------------------------------------------------------
 
diff --git a/asl_rulebook2/webapp/static/prepare.js b/asl_rulebook2/webapp/static/prepare.js
index 8b89ec8..0e536be 100644
--- a/asl_rulebook2/webapp/static/prepare.js
+++ b/asl_rulebook2/webapp/static/prepare.js
@@ -308,7 +308,7 @@ gPrepareApp.component( "download-panel", {
             <code> ./run-container.sh --data ... </code>
     </div>
     <div class="info">
-        You can edit these files directly, if you want to make changes.
+        You can edit the generated data files directly, if you want to make changes.
         <p> If you want to make changes permanent (so they happen if you redo this preparation process), check out the files in <span class="pre">$/asl_rulebook2/extract/data/</span>. </p>
     </div>
 </div>`,
diff --git a/conftest.py b/conftest.py
index 37703f8..aab14af 100644
--- a/conftest.py
+++ b/conftest.py
@@ -13,6 +13,7 @@ import pytest
 from flask import url_for
 
 from asl_rulebook2.webapp import app
+from asl_rulebook2.webapp.run_server import run_server
 from asl_rulebook2.webapp.tests.control_tests import ControlTests
 from asl_rulebook2.webapp.tests.utils import wait_for
 
@@ -140,11 +141,6 @@ def _make_webapp():
         else:
             app.config.pop( "FORCE_CACHED_SEARCHDB", None )
         app.config[ "IGNORE_MISSING_DATA_FILES" ] = True
-        # check if we will be running the prepare tests
-        if _pytest_options.enable_prepare:
-            # yup - initialize the socketio server
-            from asl_rulebook2.webapp.run_server import init_prepare_socketio
-            init_prepare_socketio( app )
         # NOTE: We run the server thread as a daemon so that it won't prevent the tests from finishing
         # when they're done. However, this makes it difficult to know when to shut the server down,
         # and, in particular, clean up the gRPC service. We send an EndTests message at the end of each test,
@@ -152,7 +148,7 @@ def _make_webapp():
         # or otherwise finish eearly before they get a chance to send the EndTests message), but we can
         # live with it.
         thread = threading.Thread(
-            target = lambda: app.run( host="0.0.0.0", port=_FLASK_WEBAPP_PORT, use_reloader=False ),
+            target = lambda: run_server( "0.0.0.0", _FLASK_WEBAPP_PORT, False ),
             daemon = True
         )
         thread.start()
diff --git a/requirements.txt b/requirements.txt
index 46c87d3..76f28c2 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,13 +1,11 @@
 # python 3.8.7
 
 flask==1.1.2
-python-socketio==5.2.1
+flask-socketio==5.1.1
+eventlet==0.33.0
 pyyaml==5.4.1
 lxml==4.6.2
 click==7.1.2
 
 pdfminer.six==20201018
 pikepdf==2.5.2
-
-# NOTE: This is needed for socketio when using the "threading" async mode.
-simple-websocket==0.2.0