The game has some weird bugs, one of which steals a lot of performance.
Wanna know what it is? The game needs to be restarted after you change any graphics settings, trust me.
After you restart the game when you changed a setting, it will recompile and this has a massive impact on GPU usage.
For example, I can enable the Ultra preset, then restart the game and put up RTSS and see I'm getting 90fps with 100% GPU usage at 4k. Then I go into the menu turn ray tracing off and jump out the menu and now I'm getting 65fps with 70% GPU usage, then I enable ray tracing again and I'm still getting 65fps with 70% usage. Then i restart the game and it's back to 90fps with 100% usage. This issue also happens when testing the FSR settings or even just messing with HDR toggles. The game should recompile everytime you change a graphics setting but it doesn't happen, it will only recompile on a restart.
So yeah, you have to restart the game after everytime you change any graphics settings otherwise you will lose as much as 50% performance
I saw people saying the get low GPU usage with a RTX4090, but once I figured out how this bug works - I get 90% to 100% usage on my RTX4090 at 4k with a 7950x CPU.