Aren't some of the buffers still based on the output resolution regardless of the render scale, at least with DLSS? Or am I remembering that wrong.interesting. turns out CP2077 at 1080p native i got ~60fps. but at 4k with performance dlss (1080p render info on debug .dll) i only got ~40fps. it seems too much for just DLSS penalty. so probably there are things that are decoupled from render resolution.
As for your performance decrease, I had the opposite happen. Patch 1.5 gave me worse performance, but I got it all back in 1.6. However, when 1.6 launched, CP2077 wouldn't for me. I did a few "verify files" and whatnot, then deleted and redownloaded the game. Turns out I just had to run it as admin, but my install is fresh so that might have something to do with it.