SUMMARY
The Cube desktop switcher effect takes a concerning amount of time to show when a skybox image is specified, regardless of size or dimensions.

I've tried with various small images ranging from a 1x1 test image to 256x small pngs, using any of those takes no less than 4 seconds for the effect to show when invoked.
Setting an actual skybox image though, which generally tend to be high definition, the effect takes even longer to start, sometimes long enough to crash Kwin.
The effect opens instantly when no skybox image path is specified or when a plain color is chosen instead.

STEPS TO REPRODUCE
1. System Settings > Desktop Effects > Window Management > Cube: Enable, Configure
2. Set Background to Skybox and choose literally any image
3. Apply and invoke the effect (Meta + C)

OBSERVED RESULT
In the presence of a skybox image, the effect takes at least 4 seconds to show, leaving the system seemingly frozen until then.
Worst case scenario (common) everything freezes and Kwin restarts after a while.

Although the actual effect seems to perform fine when it shows.

EXPECTED RESULT
Should take less time I guess, I'm very sure taking this much time for such small or even high definition images is something wrong with the effect, all other apps handle such images with ease.

SOFTWARE/OS VERSIONS
Linux: Arch
KDE Plasma Version: 6.3.5
KDE Frameworks Version: 6.14.0
Qt Version: 6.9.1