What... are you doing in the background? I've got a 3070 and 4k monitor, and I get between 50 and 60 FPS with all the settings I can fiddle with disabled enabled. I use RivaTuner to pipe statistics to a little application that drives my Logitech G19 with a real-time frame graph, CPU usage, memory load and GPU load and it uses multi cores pretty well, and generally makes use of the hardware.
-- edit --
Thanks for pointing out I made totally the wrong comment above, changing the meaning of my comment 180°...
Well I get that framerate with everything maxed at 3440x1440. I have turned things down to get a higher framerate, but it shouldn't be struggling. I don't have anything else running other than the usual programs that stay open.
Then I guess it could be a vram problem? Same res, no res scaling, 3090, no performance problems. Yes, the 90 is a bit faster but not much. But a lot more vram.