[KevinH]

What font or fonts are you using? Some fonts are broken when used with Qt. Not many but there are a few. Are you embedding fonts?

Please open the Inspector widget at the bottom of Preview and open the html tree and verify that your "i" tag is being parsed properly. That tree shows exactly how your Code is being parsed and what css is active, etc.

Please let us know what you find.

Also fwiw, I now think that these slowdowns are related to a specific kernel bug or library in Windows 10.

What exact Windows 10 update are you using and what exact build number?

According to a Google researcher, there are slowdowns and memory leaks associated with a Windows 10 security feature called Control Flow Guard that actually never frees memory in processes that open a large number of threads with different starting locations (such as the chrome browser or Sigil).

This can be especially troublesome on machines with lower amounts of memory as the only way to get the memory and speed back is to restart Windows. Restarting the impacted program will not help.

The slowdown part has been fixed in a recent Windows build. According to Google researchers Microsoft still hasn't fixed the lack of memory being freed even when an app exits (ie worse than a leak which is freed when a program exits).

The newer Qt webkits use multiple threads to walk the javascript stack to garbage collect, multiple threads to load resources (images, fonts, etc) and even multiple threads to run the gui. On my Mac, I once counted 43 different threads active when launching Sigil with a large book. Part of the reason for the large number of threads is that my cpu has 4 actual cores and therefore 8 logical hypercores.