Try running glxgears (requires mesa-demos) on a separate terminal on each monitor at the same time. I was getting ~ 60 FPS on both until I added the following to /etc/environment:
NOTE: This is only for GNOME with NVIDIA:
CLUTTER_DEFAULT_FPS=144
__GL_SYNC_DISPLAY_DEVICE=DP-2
No idea what the equivalent might be for KDE with AMD.
❯ glxgears
Running synchronized to the vertical refresh. The framerate should be
approximately the same as the monitor refresh rate.
711 frames in 5.0 seconds = 142.024 FPS
720 frames in 5.0 seconds = 143.970 FPS
720 frames in 5.0 seconds = 143.958 FPS
720 frames in 5.0 seconds = 143.977 FPS