SUMMARY
Kwin compositing appears to operate at a frame rate equal to the lowest refresh rate among all connected monitors. This means that even though my primary monitor operates at 120 Hz, everything is being forced to 75 FPS due to my second monitor operating at only 75 Hz.

Prior to the upgrade to Plasma 5.18, I was able to work around this using the following settings:

1. ForceCompositionPipeline=On in NVIDIA driver (this ensures tear-free video, always)
2. "MaxFPS=120" and "RefreshRate=120" added to ~/.config/kwinrc under the [Compositing] section (this forces the compositor to operate at 120 FPS)
3. Tearing prevention ("vsync") in kwin compositing settings set to "None"

Using these settings in Plasma 5.17, the compositor operated at a smooth 120 FPS, and NVIDIA's ForceCompositionPipeline feature ensured that there was no screen tearing on either monitor.

However, in Plasma 5.18, the above mentioned settings in kwinrc are no longer honored, and the "Show FPS" desktop effect confirms that kwin is only operating at 75 FPS. I can only get kwin to operate at 120 Hz if I disable my secondary monitor.

Please let me know if there is a workaround for this issue. Thank you.

SOFTWARE/OS VERSIONS
OS: KDE neon 5.18
KDE Plasma Version: 5.18.0
KDE Frameworks Version: 5.67.0
Qt Version: 5.13.2
Kernel Version: 5.3.0-28-generic
NVIDIA Driver Version: 440.59