Mutter is limited to 58 fps (2 frames below monitor refresh rate) on Nvidia hardware
I have a GTX 1070 so I've only just been able to use a hardware-accelerated XWayland so once I got the new driver, I tried out some games and emulators. First I tried Dolphin, BSNES, and DuckStation. All seemed to be limiting themselves to 58 fps at least most of the time. When I resize a window or mess with other parts of the UIs, the fps will generally go to 57 for a second, then up to 59 or 60 for another second, then settle at 58 fps again. I then decided to try some Steam games with Steam's FPS counter on and the same happended there.
Citra, another emulator that's actually natively supports Wayland, was able to hit and stay at 60ps. That made me think it was all XWayland related, maybe specifically with the EGLStreams back-end so I opened an issue on XWayland .
They informed me that it was likely an issue with Mutter. The only other Wayland-native applications I could think to use to test frame rate caps was Firefox so I checked that. That was also capped at 58 fps. That's why I'm here.