SUMMARY
When running kwin 5.23.2 (and all previous versions that I have tested; this doesn't appear to be a regression), I have found that moving the mouse across the edge of a window causes kwin to drop frames.  I have reproduced this on multiple systems with Intel graphics, but I don't have any other brands of GPU to test.  It happens with every version of the graphics stack I have tested, up to and including kernel 5.15.1, libdrm 2.4.107, mesa 21.3.0-rc4, and wayland 1.19.  It affects both native Wayland and xwayland windows.  It is reproducible immediately after the system is booted.

STEPS TO REPRODUCE
1. Start up a KDE wayland session
2. Open glxgears or any other application that produces smooth motion (the bug happens with all applications, but it is most noticeable with smooth movement)
3. Move the mouse over any windows border on the screen

OBSERVED RESULT
Frames are dropped, resulting in jerky motion

EXPECTED RESULT
No frames should be dropped for smooth motion

SOFTWARE/OS VERSIONS
Linux: 5.15.1
KDE Plasma Version: 5.22.3
KDE Frameworks Version: 5.87.0
Qt Version: 5.12.2

ADDITIONAL INFORMATION
I have uploaded a demonstration video to YouTube (as it is too large to attach here):  https://www.youtube.com/watch?v=ErkFgkioPeg  Please watch it in 60fps for the most effect.