So my config:
Win10
i5 12600k
32GB DDR5
RTX 3050
MSI Z690 torpedo MOBO

Until March I had stable 60-80 fps on Ultra settings.

But after a patch in the beginning of March tha game started stuttering and fps is jumping 20-70 with no relation to the settings (same issues on ultra low).

I ran some tests and found that the game uses 2 p-cores at ~40% and all 4 e-cores at 100%. Those e-cores don't give what the game needs and stuttering starts.

I disabled e-cores in BIOS and the issue was resolved - 100+ fps, no stuttering, all good.

Also I should mention that I have no such issues in other games.

It would be great if Larian could teach BG3 to use P-Cores in intel 12th CPUs so that it wouldn't be necessary to disable e-cores.

Update: found similar issue with Squad. Although it doesn't use e-cores at 100% (does at about 70%) and the game doesn't stutter, but fps is about 2 times lower than with e-cores disabled.

So I updated from win10 to win11 for the thread manager. Gotta say it's now much better with e-cores enabled: no stuttering, quite smooth. FPS is 60-100 (3070ti. 1440p. ultra).
But with e-cores disables it still is better - stable 120+ fps.