Rather than recreating QTimer objects on every update use a single one per context

Also change the timeout to 100ms which is the commonly accepted
threshold for human visual response. And stop the book details panel
debounce timer during shutdown.

The debounce timers in the popups are independent and use only data
local to the popup object so probably dont need to be shutdown.

src/calibre/gui2/__init__.py

@@ -47,6 +47,7 @@ from polyglot.builtins import iteritems, string_or_bytes
 
 del pqc, geometry_for_restore_as_dict
 NO_URL_FORMATTING = QUrl.UrlFormattingOption.None_
+BOOK_DETAILS_DISPLAY_DEBOUNCE_DELAY = 100  # 100 ms is threshold for human visual response
 
 
 class IconResourceManager:

src/calibre/gui2/dialogs/book_info.py

@@ -4,7 +4,6 @@
 
 import textwrap
 from enum import IntEnum
-from functools import partial
 
 from qt.core import (
     QAction, QApplication, QBrush, QCheckBox, QDialog, QDialogButtonBox, QGridLayout,
@@ -16,6 +15,7 @@ from qt.core import (
 from calibre import fit_image
 from calibre.db.constants import RESOURCE_URL_SCHEME
 from calibre.gui2 import NO_URL_FORMATTING, gprefs
+from calibre.gui2 import BOOK_DETAILS_DISPLAY_DEBOUNCE_DELAY
 from calibre.gui2.book_details import (
     create_open_cover_with_menu, resolved_css, details_context_menu_event, render_html, set_html,
 )
@@ -24,8 +24,6 @@ from calibre.gui2.widgets import CoverView
 from calibre.gui2.widgets2 import Dialog, HTMLDisplay
 from calibre.startup import connect_lambda
 
-BOOK_DETAILS_DISPLAY_DELAY = 250 # 250ms is arbitrary
-
 
 class Cover(CoverView):
 
@@ -225,7 +223,10 @@ class BookInfo(QDialog):
         self.path_to_book = None
         self.current_row = None
         self.slave_connected = False
-        self.slave_debounce_timer = QTimer()
+        self.slave_debounce_timer = t = QTimer(self)
+        t.setInterval(BOOK_DETAILS_DISPLAY_DEBOUNCE_DELAY)
+        t.setSingleShot(True)
+        t.timeout.connect(self._debounce_refresh)
         if library_path is not None:
             self.view = None
             db = get_gui().library_broker.get_library(library_path)
@@ -349,13 +350,11 @@ class BookInfo(QDialog):
         QTimer.singleShot(1, self.resize_cover)
 
     def slave(self, mi):
-        self.slave_debounce_timer.stop()
-        t = self.book_display_info_timer = QTimer()
-        t.setSingleShot(True)
-        t.timeout.connect(partial(self._timed_slave, mi))
-        t.start(BOOK_DETAILS_DISPLAY_DELAY)
+        self._mi_for_debounce = mi
+        self.slave_debounce_timer.start() # start() will automatically reset the timer if it was already running
 
-    def _timed_slave(self, mi):
+    def _debounce_refresh(self):
+        mi, self._mi_for_debounce = self._mi_for_debounce, None
         self.refresh(mi.row_number, mi)
 
     def move(self, delta=1):

src/calibre/gui2/library/views.py

@@ -18,7 +18,9 @@ from qt.core import (
 
 from calibre import force_unicode
 from calibre.constants import filesystem_encoding, islinux
-from calibre.gui2 import FunctionDispatcher, error_dialog, gprefs
+from calibre.gui2 import (
+    BOOK_DETAILS_DISPLAY_DEBOUNCE_DELAY, FunctionDispatcher, error_dialog, gprefs,
+)
 from calibre.gui2.dialogs.enum_values_edit import EnumValuesEdit
 from calibre.gui2.gestures import GestureManager
 from calibre.gui2.library import DEFAULT_SORT
@@ -27,9 +29,9 @@ from calibre.gui2.library.alternate_views import (
 )
 from calibre.gui2.library.delegates import (
     CcBoolDelegate, CcCommentsDelegate, CcDateDelegate, CcEnumDelegate,
-    CcLongTextDelegate, CcMarkdownDelegate, CcNumberDelegate, CcSeriesDelegate, CcTemplateDelegate,
-    CcTextDelegate, CompleteDelegate, DateDelegate, LanguagesDelegate, PubDateDelegate,
-    RatingDelegate, SeriesDelegate, TextDelegate,
+    CcLongTextDelegate, CcMarkdownDelegate, CcNumberDelegate, CcSeriesDelegate,
+    CcTemplateDelegate, CcTextDelegate, CompleteDelegate, DateDelegate,
+    LanguagesDelegate, PubDateDelegate, RatingDelegate, SeriesDelegate, TextDelegate,
 )
 from calibre.gui2.library.models import BooksModel, DeviceBooksModel
 from calibre.gui2.pin_columns import PinTableView
@@ -1615,16 +1617,20 @@ class BooksView(QTableView):  # {{{
             self._model.search_done.connect(self.alternate_views.restore_current_book_state)
 
     def connect_to_book_display(self, bd):
-        self.connect_to_book_display_timer = QTimer()
-        self._model.new_bookdisplay_data.connect(partial(self._timed_connect_to_book_display, bd))
-
-    def _timed_connect_to_book_display(self, bd, data):
-        self.connect_to_book_display_timer.stop()
-        t = self.connect_to_book_display_timer = QTimer()
+        self.book_display_callback = bd
+        self.connect_to_book_display_timer = t = QTimer(self)
         t.setSingleShot(True)
-        t.timeout.connect(partial(bd, data))
-        from calibre.gui2.dialogs.book_info import BOOK_DETAILS_DISPLAY_DELAY
-        t.start(BOOK_DETAILS_DISPLAY_DELAY)
+        t.timeout.connect(self._debounce_book_display)
+        t.setInterval(BOOK_DETAILS_DISPLAY_DEBOUNCE_DELAY)
+        self._model.new_bookdisplay_data.connect(self._timed_connect_to_book_display)
+
+    def _timed_connect_to_book_display(self, data):
+        self._book_display_data = data
+        self.connect_to_book_display_timer.start()
+
+    def _debounce_book_display(self):
+        data, self._book_display_data = self._book_display_data, None
+        self.book_display_callback(data)
 
     def search_done(self, ok):
         self._search_done(self, ok)

src/calibre/gui2/ui.py

@@ -1191,6 +1191,8 @@ class Main(MainWindow, MainWindowMixin, DeviceMixin, EmailMixin,  # {{{
 
     def shutdown(self, write_settings=True):
         self.shutting_down = True
+        if hasattr(self.library_view, 'connect_to_book_display_timer'):
+            self.library_view.connect_to_book_display_timer.stop()
         self.shutdown_started.emit()
         self.show_shutdown_message()
         self.server_change_notification_timer.stop()