I'm also on Emacs and Niri, seems to be a popular combo, and I don't have any performance problem.

At home I'm driving an Ultrawide (3440x1440@75Hz) at scale 1.1.

At work I'm driving two 4k screens at scale 1.2.

I might be less sensitive to latency but it could also be a graphics driver issue or something similar. I'm using Arch with emacs-wayland (pgtk) with a strix halo (all AMD) laptop.