You can try Gamescope [1] from Valve, that's what Steam Deck uses - i think its a compositor designed to minimize latency but support the few things games need. Some compositors like KDE Plasma KWin support a direct scanout mode which is the same idea as windows' exclusive fullscreen. You might need to look for support for something similar in niri.
As far as I know niri enables direct scanout by default. It's an option you can disable if you want https://niri-wm.github.io/niri/Configuration%3A-Debug-Option.... I do not have this set which indicates direct scanout is enabled.
It's interesting because the latency is only when pressing keys on the keyboard. Mouse movement and button press latency feels as good as Windows, I can't perceive any delay. I tried 3 keyboards, it's all the same. I'm also not running anything like keyd or anything that intercepts keys. It's a vanilla Arch Linux system on both of the systems I tested.
That makes it unlikely it's related to this auto suspend, especially since it only appears to happen when I use my 4k monitor, but I did set it to 30s just to rule it out and it made no difference. I brought it back to 2s afterwards.
Windows's desktop compositor DWM is actually very advanced, and I don't believe any Linux desktop compositor is anywhere close. It's one of the things I miss when leaving Windows.