E-book viewer: Fix a regression that caused an increase in load times when loading the individual HTML files inside an EPUB book. Fixes #1658578 [loading flow delay for every page](https://bugs.launchpad.net/calibre/+bug/1658578)

src/calibre/gui2/viewer/fake_net.py

@@ -6,7 +6,7 @@ from __future__ import (unicode_literals, division, absolute_import,
                         print_function)
 import os
 
-from PyQt5.Qt import QNetworkReply, QNetworkAccessManager, QUrl, QNetworkRequest, QTimer, pyqtSignal
+from PyQt5.Qt import QNetworkReply, QNetworkAccessManager, QUrl, QNetworkRequest, QTimer, pyqtSignal, QByteArray
 
 from calibre import guess_type as _guess_type, prints
 from calibre.constants import FAKE_HOST, FAKE_PROTOCOL, DEBUG
@@ -19,6 +19,9 @@ def guess_type(x):
     return _guess_type(x)[0] or 'application/octet-stream'
 
 
+cc_header = QByteArray(b'Cache-Control'), QByteArray(b'max-age=864001')
+
+
 class NetworkReply(QNetworkReply):
 
     def __init__(self, parent, request, mime_type, data):
@@ -30,6 +33,7 @@ class NetworkReply(QNetworkReply):
         self.__data = data
         self.setHeader(QNetworkRequest.ContentTypeHeader, mime_type)
         self.setHeader(QNetworkRequest.ContentLengthHeader, len(self.__data))
+        self.setRawHeader(*cc_header)
         QTimer.singleShot(0, self.finalize_reply)
 
     def bytesAvailable(self):

src/calibre/gui2/viewer/main.py

@@ -954,6 +954,7 @@ class EbookViewer(MainWindow):
         while worker.isAlive():
             worker.join(0.1)
             QApplication.processEvents()
+        self.view.settings().clearMemoryCaches()
         if worker.exception is not None:
             tb = worker.traceback.strip()
             if tb and tb.splitlines()[-1].startswith('DRMError:'):