[KevinH]

Okay, I could not find the issue in calibre, but a general web search indicates the Windows has an "efficiency mode", that will automatically lower the priority of programs it doesn't like for some reason.

To rule this out, fire up 2 or 3 instances of Sigil and go to Windows task manager, and disable efficiency mode for the second and third instances of Sigil and all its threads and test again.

Please include screenshots showing the priority of all threads generated by Sigil across all instances, including the multiple QWebEngineProcess thread.

As all of this works just fine on MacOS, so I will test with multiple instances of Sigil on Linux to see how fast it is.

My bet is this is truly related to Windows lowering priority for memory hungry processes like browsers for its efficiency mode (that is triggered by the new Qt WebEngine), but I do not have a Windows box to test that hypothesis with.

As an temporary workaround, try using the template plugin to load commonly used code when starting a new epub or simply use FolderOut to save commonly used epub pieces to a folder which can easily be loaded by any instance of Sigil.

If someone who can recreate this Preview slowdown on Windows with multiple instances of Sigil, please use the task manager to see if the second or third instances and their threads have had their priorities lowered. Please let us know what you see.