potentially a resource cap, I’d try running the game at a low resolution and low settings and see if it resolves it. When either system RAM or VRAM max out you can get severe stuttering in games. Otherwise with iMacs it could be a lot more complex to troubleshoot if that doesn’t work.
If you can run the game at like 600x400 at the lowest settings, then dial up the resolution till it starts again, if it doesn’t when you’re at native resolution, then dial up the settings.