Again, and as with @bobnewels, disable your e-cores (assuming that video is your system). SC doesn't like them at all (a reasonably common issue with e-cores and games, I hear). For comparison, at the 10-minute mark of that video going up the stairs where you're getting 25fps, I get 35fps there -- using a very old Intel Sandybridge 3970X 6-core/12-thread CPU and a 1080Ti, and that's on a full server. I'm at 3440x1440 not 4K however, but SC is CPU-bound not GPU-bound, which we can see with your 3090Ti being underutilised. Disable the e-cores and you should get a big jump. Also, the stuttering in the video are the shaders being compiled. This only happens the first time you visit an area you haven't been to before, after that it's perfectly smooth. Just make sure you set your Nvidia Shader Cache to at least 5GB. Currently, if you explore all of Stanton, the cache will get to around 4.2GB, but as the cache is shared with other games, you could easily go past 5GB. I set mine to 10GB.