Please also check this post related to slowdowns seem,y related to Sigil 1.5.1 but was not ...

https://www.mobileread.com/forums/sh...&postcount=102

It turns out their back-up software was causing the problems. Please verify that neither AV nor backup software like that case is causing your issues.

Fortunatly when I upgraded from 1.3.x to 1.6.0 the problem has disappeared. No closer to a solution but at least it's fixed.

Yes it would probably be best to test with the latest Sigil-1.6.0.

So far slowdowns have been tracked back to:
- AV software
- Third party utilities that scan files as part of its "security" just like AV (some back-up software)
- Setting lower power consumption settings on computer
- a recent Windows 10 update that impacted video drivers
- old video drivers or missing opengl implementations
- conflicts with Qt based system level utilities.