[KevinH]

Okay, I found a test epub that had 2500 junk xhtml files that I used for my rename testing. It takes a count to about 3 or 4 seconds to do a full save as on my lowly macOS laptop.

@DiapDealer, I emailed it to you directly in case you want it for testing.

With it under MacOS (which shares one process unlike Windows and Linux), the second instance of Sigil was unresponsive while the first version did its save as. But this is expected behaviour on Mac OS since they share all worker processes and even the main gui process.

I then fired up Linux in a multi-threaded VM and tried the exact same test. Linux uses completely separate processes for multiple instances of Sigil.
Linux allowed me to easily switch to the second instance of Sigil and start editing while the first did the save as in the background. Again, this is what I would expect for Linux and Windows.

I have no idea why your Windows box with fully separate main processes and separate worker threads does what it does. It is either something specific to Windows or to your set-up.

But this demonstrated that there is nothing inherent in Sigil's code that would force a clash or make things unresponsive on Windows and Linux -- just MacOS due to its inherent multi-mainwindow design on that platform.

So please use your task manager to turn off Efficiency Mode for all threads associated with both copies of Sigil before trying your test. Please let us know what you find.