That's actually really interesting, and like you said, I would've also definately believed VaM to be more cpu bound than gpu bound, but looking at these benchmarks like you mentioned, the 5090 actually does make quite a difference. I am running CL30 6000mhz ram if that makes any difference...
Adding this here in case someone is curious, or looking to upgrade, or maybe just wants to compare.
At 4K resolution, this is how an RTX 5090 + 9800X3D performs on this benchmark. (I am on windows 11, but it detected as 10 for some reason, if it matters)