diff --git a/src/calibre/gui2/viewer/web_view.py b/src/calibre/gui2/viewer/web_view.py
index 6c7298d564..bd2c10dcea 100644
--- a/src/calibre/gui2/viewer/web_view.py
+++ b/src/calibre/gui2/viewer/web_view.py
@@ -229,6 +229,7 @@ def create_profile():
 class ViewerBridge(Bridge):
 
     view_created = from_js(object)
+    on_iframe_ready = from_js()
     content_file_changed = from_js(object)
     set_session_data = from_js(object, object)
     set_local_storage = from_js(object, object)
@@ -499,6 +500,7 @@ class WebView(RestartingWebEngineView):
         self._page.linkHovered.connect(self.link_hovered)
         self.view_is_ready = False
         self.bridge.bridge_ready.connect(self.on_bridge_ready)
+        self.bridge.on_iframe_ready.connect(self.on_iframe_ready)
         self.bridge.view_created.connect(self.on_view_created)
         self.bridge.content_file_changed.connect(self.on_content_file_changed)
         self.bridge.set_session_data.connect(self.set_session_data)
@@ -628,6 +630,9 @@ class WebView(RestartingWebEngineView):
         for func, args in iteritems(self.pending_bridge_ready_actions):
             getattr(self.bridge, func)(*args)
 
+    def on_iframe_ready(self):
+        performance_monitor('iframe ready')
+
     def on_view_created(self, data):
         self.view_created.emit(data)
         self.view_is_ready = True
diff --git a/src/pyj/read_book/view.pyj b/src/pyj/read_book/view.pyj
index 057d20354c..3f7f8902fb 100644
--- a/src/pyj/read_book/view.pyj
+++ b/src/pyj/read_book/view.pyj
@@ -769,6 +769,8 @@ class View:
 
     def on_iframe_ready(self, data):
         data.width, data.height = self.iframe_size()
+        if ui_operations.on_iframe_ready:
+            ui_operations.on_iframe_ready()
         return self.do_pending_load
 
     def do_pending_load(self):
diff --git a/src/pyj/viewer-main.pyj b/src/pyj/viewer-main.pyj
index adeac66e34..30aecaeff4 100644
--- a/src/pyj/viewer-main.pyj
+++ b/src/pyj/viewer-main.pyj
@@ -431,6 +431,8 @@ if window is window.top:
         to_python.show_book_folder()
     ui_operations.show_help = def(which):
         to_python.show_help(which)
+    ui_operations.on_iframe_ready = def():
+        to_python.on_iframe_ready()
 
     document.body.appendChild(E.div(id='view'))
     window.onerror = onerror