Are you joking? It's absolutely i/o related. Hell, even Microsoft says this. It's one of the core tenants of the #VelocityArchitecure. I've never heard someone argue it's not and I'm surprised it's coming from you.
I'm not talking about SFS-like programs I'm talking about the supposed bespoke hardware in Xbox whichever PC GPUs that supposedly works in tandem with SFS. Partially resident textures which has been around for some time now. The Xbox supposedly has hardware in GPU to better manage transition from low mip to high in the event that a request from the GPU isn't loaded in time into memory. It's a backstop for any latency from disk to VRAM causing the higher mip to not load in time.
Simply put, this should never happen on PS5.
Also, Nanite was the key feature of UE with it's ability to do software rasterization much faster/more efficient than GPU, which is why I say mesh shader isn't so amazing now. The ability to push 8k textures had more to do with PS5 decompression. Brian Karis admitted so himself.