SUMMARY
The monitor's refresh rate does affect KDE's notifications and plasmoid widgets "tick" speed.

STEPS TO REPRODUCE
1. Have a monitor set to 240 Hz in settings. I guess it happens with every value >60 Hz.
2. Log in into session (If already logged in, end session and log back in.)
3.1 Run "notify-send -i 5000 'Hey\!' 'This should last 5 seconds'" for testing notification.
3.2 Play Spotify or some other compatible media and have the "Media Player" widget on the desktop for testing plasmoid widgets.

OBSERVED RESULT
Monitor refresh rate does affect notification and plasmoid tick speed. Notifications disappear too fast and plasmoid widgets tick too fast (especially noticeable with the Media Player widget, it's not in sync with the actual media, is way too fast)

EXPECTED RESULT
Monitor refresh rate should not affect notification and plasmoid widget tick speed.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux (Kernel 5.11.11) / KDE Plasma 5.21.4
KDE Plasma Version: 5.21.4
KDE Frameworks Version: 5.80.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
X.Org Version: 1.20.10
Drivers: Nvidia version 460.67
Compositor backend: OpenGL 3.1
Compositor Scale Method: Smooth
Compositor Latency: Balance of latency and smoothness
2 monitor setup, both 2560x1440, left-hand side one @60 Hz, right-hand side primary one @240 Hz

Demonstration: https://imgur.com/a/pjCpU3h (last one is Spotify + Media Player Widget @240 Hz, imgur didn't apply the description, or I forgot it)