GPU-bound stuff is largely unaffected; CPU-bound definitely takes a hit (although there's no noticeable additional latency on non-RT tasks), but that's kinda to be expected.
I would not expect lower FPS, because the amount of available CPU does not materially change. I would expect higher latency, because RT threads would more often scheduled ahead of other threads.
GPU-bound stuff is largely unaffected; CPU-bound definitely takes a hit (although there's no noticeable additional latency on non-RT tasks), but that's kinda to be expected.