@Sven - thanks.
I did more tests with various Mesa and LO versions. Mesa doesn’t seem to be the culprit: with the gtk3 VCL plugin, LO v7.0.4.2 doesn’t lag on R4.1 on debian-11 and debian-12; but v7.3.7.2 is unusable on f36 (and f37, and debian-12 with the debian’s LO debs, and debian-12 with LO debs, etc.).
→ will need to bisect LO versions to see which one introduced the issue.
[edit: the bug was introduced in 7.1.0.0.alpha1 ; 7.0.6.2, the previous version, is OK.]
[edit2 / 2023-05-15: bug filed: 155326 – Calc with gtk3 VCL is unusable with large spreadsheets: huge lag, high CPU usage, etc.]
[edit3 / 2023-06-07: found the commit that introduced the issue - comment]
tests
The main differences between R4.0 and R4.1 are Mesa versions: 17x vs 20x on dom0, while debian stable is on 20.x and fedora 37 on 23.x.
On my T450s with R4.1 I have accelerated graphics, but on my 13th gen hw R4.1 isn’t recent enough so no acceleration yet (=llvmpipe). Mind you, I just found this out and hadn’t even noticed any lag.
That would rule out acceleration in dom0 as a contributing factor to LO’s high CPU usage with VCL=gtk3.
Comparing templates:
on debian-11 minimal template, apt install libreoffice-calc mesa-utils.
→ LO uses the x11 VCL by default (same as SAL_USE_VCLPLUGIN=gen) so it’s blazing fast.
then, apt install libreoffice-gtk3=1:7.0.4-4+deb11u6.
→ LO was a bit slower than x11/gen but still much, much faster than LO on my f37 template: scrolling stopped immediately after releasing arrow keys, while on f37 scrolling went on for at least a few seconds.
OK - so debian-11 with LO 7.0.4.2 works on R4.1 whatever the VCL plugin.
Same test as above with a debian-12 ITL testing template (shipping with mesa: 22.x): LO is unusable, like in f36/f37 templates.
But LO version in debian-11 is 7.0.4.2; in f37 it’s 7.4.6.2; in debian-12 it’s 7.4.5.1.
Could it be that LO official releases (/debs) work while distro-packaged ones dont?
→ install libreoffice 7.0.4.2 from LO’s archive debs on debian-12: no lag.
→ install libreoffice 7.4.5.1. from LO’s archive debs on debian-12: lag.
So, it’s not an issue with distribution packages; LO seems to have introduced the bug somewhere between those versions.