On KWin's Wayland session, the framerate inside XWayland applications gets limited to the monitor's refresh rate when either setting the window to become fullscreen or removing its borders through the context menu. This can be seen with glxgears. 

STEPS TO REPRODUCE
1. Run "env vblank_mode=0 __GL_SYNC_TO_VBLANK=0 glxgears"
2. Remove the window borders with the context menu (Alt-F3->More Actions->No border).


OBSERVED RESULT
The framerate is uncapped until the window borders get removed, then it drops to the monitor refresh rate.

EXPECTED RESULT
Uncapped framerate

SOFTWARE/OS VERSIONS
OS: openSUSE Tumbleweed
Linux Kernel: 5.17.4-1 
KDE Plasma Version:  5.24.4
KDE Frameworks Version: 5.93.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
I also tested Mutter and wlroots (Wayfire) to reproduce the issue there, both through a standalone wayland session and nested inside KWin itself, but both work as expected.