Sounds like you're using an AMD card. AMD drivers are known to be awful with OpenGL rendering. AMD users have struggled for years.
They released an optional driver a while back which seemed to fix almost all slow downs.
https://www.amd.com/en/support/kb/release-notes/rn-rad-win-22-7-1Later drivers were crap again. I've given up trying the very latest drivers. It's too much trouble downloading big files, installing, then uninstalling immediately because the performance is bad.
If you're not happy using old drivers, you can always run Supermodel with -legacy3d. Emulation isn't as accurate, but it's not affected by slow downs.
Just make sure the next card you get is Nvidia.
BTW.
All these user guides that suggest the correct way to install Supermodel is to first download it from a foreign web site, aren't worth watching.
Use official downloads
https://www.supermodel3.com/Download.html