SUMMARY
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Kwin constantly skips frames during & after window resizing. Since maximizing windows also results in resizing, frame skip occurs in that case too. I have tested this using glxgears.
There is no frame drop as reported by glxgears, but visibly, the animation gets paused for around 500ms after resizing a window. This skip is also visible on videos playing in VLC. Hence, it seems that kwin is skipping some frames after resizing windows.
Resizing CSD windows doesn't cause this problem.

STEPS TO REPRODUCE
1. Open any application (non-csd)
2. Open glxgears.
3. Resize the window/maximize it.
3. Observe glxgears pause for a few moment.

OBSERVED RESULT
Kwin skips some frames for the whole desktop after resizing any window.

EXPECTED RESULT
Kwin should display every frame without skipping any.

SOFTWARE/OS VERSIONS
Linux: 5.15.70-1-lts
(available in About System)
KDE Plasma Version: 5.25.5
KDE Frameworks Version: 5.98.0
Qt Version: 5.15.6

ADDITIONAL INFORMATION
GPU: Mesa Intel® HD Graphics 2500