Haven't we already determined that Qt6.9.0 has introduced some undesirable behavior and sluggishness in Sigil's QtWebEngine rendering on Linux?? We can't really do much there. There were lots of changes to QtWebEngine between the 6.8 and 6.9 series. Best case is to hope that Qt6.9.1 addresses some of it when it's released (and Arch incorporates it). I'm not happy with Sigil's performance on my Arch machine either. That's why I' mostly using the AppImage until it gets fixed.
I'm experimenting with environment variables to see if something can affect the rendering engine in a positive way. From what I can see, the QtWebEngine rendering in Qt6.9.0 immediately drops to Vulcan software rendering based solely on the video vendor being "nvidia".
It is my understanding that any of the proprietary Nvidia drivers after version 495.44 (which my drivers are way beyond) includes a GBM backend. Yet the webengine included in Qt6.9.0 automatically complains that "GBM isn't supported in my configuration" based on nothing but the vendor name "nvidia". *shrug*
See Peter Varga's comment here:
https://bugreports.qt.io/browse/QTBUG-136160
No one's explanation for why this is being done satisfactorily explains to me why the behavior changed between 6.8 and 6.9. Qt suggests the
change occurred in Qt6.6, yet Sigil worked just fine on my Arch machine with Qt6.6, 6.7, and 6.8. No idea why they decided to break video webengine rendering performance in 6.9 for millions of users.