SUMMARY
When a Firefox picture-in-picture window is open and a timed notification comes on screen, the video is affected by terrible lag until the notification is gone or manually closed.

Not sure if this is even a KDE thing or a Firefox thing (but I don't see the problem on Firefox for Windows). Tested in LibreWolf too and it's the same.

It's reproducible every time (version 5.27 too).

STEPS TO REPRODUCE
1. Have a video playing in picture-in-picture mode in Firefox.
2. Wait for a notification to appear (KTeaTime notifications work every time to trigger this issue; but only the timed one when the tea is ready; not the one when the timer starts)

OBSERVED RESULT
While the temporary notification is visible, the video will lag a lot. Once the notification automatically closes, playback will continue normally. It will happen again on the next notification.

EXPECTED RESULT
Notifications do not affect video playback.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Neon
KDE Plasma Version: 6.0.0
KDE Frameworks Version: 6.0.0
Qt Version: 6.6.2
Using X11

ADDITIONAL INFORMATION
NVIDIA card, proprietary drivers; AMD CPU.
Notifications set to bottom right corner, if it matters. Single monitor.
Using standalone notifications widget in the taskbar instead of the entry in the system tray, which is deactivated.
This issue is 100% reproducible every single time and I have experienced it for as long as I can remember.
Apologies if this does not belong here; but I assumed it's worth a shot.