This seems to be game dependent. In Ratchet and Clank, even at 4K, the average frame rate on the 5090 is virtually the same with DS on. You lose about 9% in 1% lows, but that's acceptable since you are getting higher quality textures (you can see at 4K that if you pause at certain points, DS off has blurrier textures.)Nothing on that was said anywhere and there are no signs of that in whatever little performance testing we have.
5090 is still losing performance with DS when ran in high GPU load scenario (4K res). As I've said already this isn't any different to how Lovelace handle this - if you're CPU limited then GPU decompression is "free" (obviously).
Whereas in Spider-Man 2, a game that is CPU bound to begin with, you lose performance with DS enabled even at 1080p. This happens on every NVIDIA GPU, just less so on the 5090.
Someone should test Ratchet and Clank at lower resolutions on a 4090. From what I remember, it still lost performance with DS enabled even at 1440p and 1080p, but I can't find any tests online.