The 3090 + 5900x is a mistake. The 5900x is 2 x 5600x CPUs. So therefore, when the games asks for 8 cores, it will get 6 good cores and 2 very slow cores across the infinity switching fabric. What's more, NVidia GPUs take MUCH MORE CPU than AMD GPUs. You should either buy an AMD GPU or upgrade/downgrade to ANYTHING OTHER THAN 5900x with 8+ cores (5800x, 5800, 5700, 5700x3d, 5950x, 5900xt, anything really ...)
I've been looking hungrily at a 5700x3d, but just can't justify spending more money on AM4. I'm saving up for a 9800x3d.
I've considered just disabling a CCD to see how that works.