Same issue here, on the RTX 4080 (driver version 537.58), i9-13900K, 64GB RAM. Like he described, it's happening even in the menus. If I enable the ingame FPS Counter, it shows wildly jittering framerates between 70-144fps in the main menu without actions, when it was always hitting the target 144fps in any menu before.
EDIT1: Disabling all new features (back to MSAA 4x, no Supersampling, no SSS, etc.; basically same settings as in 2.8) does not fix the issue.
EDIT2: I just experimented switching back from the multi-threaded executable in 'bin-mt' to using the one in the previous 'bin' folder. The issue completely disappears using the ST executable, but of course I'd prefer using the new MT version. Any ideas what could be causing this?
EDIT3: A friend recommended trying to use MT but with disabled HyperThreading, which supposedly can cause this issue.
I cannot confirm this, after disabling HyperThreading, the issue on the MT version persists, the frequency of stutters and brightness changes might have changed, this however could be subjective. There is a definitive impact when disabling SSAO and SSLR, lowering the frequency of spikes down to about 25% of before (every 2s instead of every 0.5s) but it's still unplayable.
ST version seems unaffected by HT on/off. Reenabled HyperThreading and did more testing: Performance of ST is far superior for me now since 2.9 (no stutter at all, smooth framerates), this was not the case in 2.8 and before - the MT version was my preferred option until now.