SUMMARY
When switching wallpapers, either in System Settings or by right-clicking on the desktop and selecting "Desktop and Wallpaper", there is a fade transition. This animation speed depends on the monitor refresh rate. For example it will move faster at 120Hz than at 60Hz.

I noticed this on my desktop PC, but will also check on my Steam Deck (Plasma 5 X11) and my laptop (Plasma 6 Wayland), to test both session types and different screen arrangements (my laptop is a single display).

STEPS TO REPRODUCE
1. Open one of the Wallpaper configuration screens.
2. Select a new Wallpaper.
3. The fade animation will be faster on higher refresh rate displays.

OBSERVED RESULT
The fade animation speed is tied to the display's refresh rate. 

EXPECTED RESULT
The fade animation should be framerate-independent.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 6.1.4
KDE Frameworks Version: 6.5.0
Qt Version: 6.7.2
Kernel Version: 6.10.7-zen1-1-zen (64-bit)
Graphics Platform: Wayland
Processors: 16 × AMD Ryzen 7 3700X 8-Core Processor
Memory: 31.3 GiB of RAM
Graphics Processor: AMD Radeon RX 7900 XTX

ADDITIONAL INFORMATION
This is not advocating for a preference in speed, though I prefer the faster animation speed, this is a report about the inconsistency.