I'm not so sure that Nouveau is slower than the proprietary Nvidia driver. I didn't run benchmarks on my personal use case but my subjective experience is that Nouveau might be faster. It's a Debian 11, X11, NVIDIA driver vs Debian 13, X11, Nouveau on the same laptop with a Quadro K1100mq. The desktop of the newer system seems to be faster. Of course it could be the sum of the individual improvements of kernel, GNOME, etc. I only move windows around my desktop, no games, so it's a very limited scenario.

Absolutely not. Nouveau might give you a usable desktop but the second you need to do any 3d rendering or decoding it’s atrocious.

In my experience, the proprietary driver has always blown away nouveau at 3D rendering performance and featureset.