Upgraded to a 7950X and 4090 recently and just tested stuff in VRChat, namely vket worlds where I always had FPS issues on my old hardware. Mostly played around with settings trying to keep my FPS capped at 90.

From what I can tell AA is the worst offender on FPS by far. Graphic preset seems to matter way less. Ultra with no AA was a guaranteed 90 FPS no matter what, everywhere. Ultra x2 or High x4 are the next best tier, being a capped 90 most of the time with very very rare geography causing dips to 65-70 (pretty much only one whole object I found in any vket world caused this at high x4, it was solid 90 otherwise). Unfortunately without AA the game is just way too ugly, and x2 doesn't really cut it, so x4 is the minimum, which makes Ultra a no go. High x4 was the one I had the most success with for a balance of good AA with a 99.9% uptime on solid 90FPS. Interestingly going from High to Medium always seems to make the FPS WORSE, no idea why, with the only way to get any further improvement being to go to Low, where everything looks like shit. So High x4 is definitely the sweet spot here.

Of course this might all go out the window this summer when we get the new corporate worlds, as these are really the ones that stress hardware. This winter's worlds in particular were really nasty for my old PC. But in the worst case scenario, turning off or lowering AA seems to always be the most reliable way to increase performance. I imagine even the corporate worlds would be a stable 90 FPS at Ultra just fine as long as you're willing to turn AA off completely. My guess is High x4 will be fine 90% of the time with some occasional small drops at the usual problematic booths.

This was done on a Reverb G2, by the way, so my resolution is a bit higher than the Index. Also, on my old PC I had that really annoying graphical bug when opening the steam overlay within VRChat. This doesn't happen anymore on my new build, or at least I can't get it to happen.