SUMMARY
opening the plasma style settings page takes more than 30s, when it contains certain themes (numix, arc color, 'Red-Dot-Black-Plasma')

STEPS TO REPRODUCE
1. get a new plasma style via the get new plasma styles button, most of them are affected
2. close systemsettings
3. re-open systemsettings, and navigate to Appearance and then 'Plasma Styles'

OBSERVED RESULT
very long (>30) loading time, high single core usage 

EXPECTED RESULT
no loading time

Operating System: Arch Linux
KDE Plasma Version: 5.20.90
KDE Frameworks Version: 5.78.0
Qt Version: 5.15.2
Kernel Version: 5.10.10-zen1-1-zen
OS Type: 64-bit
Processors: 8 × AMD Ryzen 5 2500U with Radeon Vega Mobile Gfx
Memory: 15,2 GiB of RAM
Graphics Processor: AMD Radeon™ Vega 8 Graphics
ADDITIONAL INFORMATION:

this also happens with the cursors and window deco page, and probably other pages as well

I've already run into this issue, before installing plasma 5.21beta, looked like only the cursor page was affected