I don't know what it is with this patch but it's just downright terrible for FPS. Other players are also reporting the same when I asked on the Discord about it.
For context on Stable I get about 90 fps in Cherno with my settings in the high-extreme range.
On Experimental I'm dropping into the low 20s at times. It's horrible.
Get a few players together and performance drops even further, even on Stable.