I wish they included the window compositor as something that can introduce latency because I'd like to learn more about it.

When I switched from Windows to Linux on the same hardware I noticed a lot of keyboard input latency when playing games, at least 150ms. This only happens to me with niri, KDE Plasma (Wayland) feels identical to Windows. So did Hyprland. I'm able to reproduce it on multiple systems when I have a 4k display running at 1:1 native scaling. On AMD cards, turning off v-sync helped reduce it but it didn't remove it. With an NVIDIA card, turning off v-sync made no difference. I believe it's semi-related to that 4k display because when I unplug that display and use my 2560x1440 monitor, it's much less noticeable despite getting a solid 60 FPS with both monitors. All that to say, there's certainly a lot more than your input device, GPU and display playing a role.

If anyone played Quake on a dial-up connection with client side prediction turned off, that is the exact same feeling. It's pressing a key and then seeing the screen update X ms afterwards.