Nvidia 5090 awful performance

Nvidia 5090, brand new cutting-edge everything pc. The game runs TERRIBLY. It's mostly okay in a map solo with 1 or 2 packs on the screen. The moment I group up with a friend (added effects) or get in some situation with lots of mobs + effects on the screen it's a complete slide show. You just have to guess where anything is or what's actually happening.

I was getting crashes on day 1 of the season, but those seem to be gone now.

This might be very basic, but never hurts to ask:
-Have you tried deleting the Shader folders?
-Have you tried re-installing the game?
-Have you tried updating the graphics card?
-Have you tried switching between Direct 11, 12 and Volkan?
-Doubt this would be the issue, but from personal experience it can F quite a bit with the performance: Have you tried switching Vsync on/off?

Path of Exile's an ARPG. It's much more likely to be CPU-bound than GPU-bound.

What does the rest of your system look like? What resolution etc do you play at?

Thank you for the suggestions. I tweaked a few things. Turned out the biggest thing was that multi-threading was disabled for some reason. Not something I clicked. I think that made all the difference.