SUMMARY
On my 150% scaled display, moving the mouse over KOrganizer's agenda view makes it take noticeably long to update, most visible by the red line on the timeline that shows where the mouse is moving like the mouse was moved but delayed. It seems to process every mouse move event even if new ones have already come in which makes it take a long time to become responsive again if you moved the mouse around a lot.

The issue occurs similarly but way less severely when moving KOrganizer over to my 100% scaled display. There, just moving the mouse around makes it behave normally but dragging to select cells still has the same problem. (I'm assuming it's just not taking as long to redraw the window there so it's less noticeable.)

I put "Wayland (?)" in the title because I'm not exactly sure if this is Wayland-specific, I noticed it first when I switched to Wayland though and haven't tried X11 since.

STEPS TO REPRODUCE
1. (optional) Set display scale to 150%
2. Open agenda view
3. Move/drag mouse around on the table

OBSERVED RESULT
The UI slows down, taking longer to regain responsiveness the more you moved the mouse around.

EXPECTED RESULT
The UI stays responsive.

SOFTWARE/OS VERSIONS
Operating System: Gentoo 2.8
KDE Plasma Version: 5.23.80
KDE Frameworks Version: 5.88.0
Qt Version: 5.15.2
Kernel Version: 5.14.14-gentoo-dist (64-bit)
Graphics Platform: Wayland
Processors: 12 × AMD Ryzen 5 5600X 6-Core Processor
Memory: 23.4 GiB of RAM
Graphics Processor: Radeon RX Vega

ADDITIONAL INFORMATION
KOrganizer commit 7ab02ec