Yeah, I encountered this the moment I tried jumping on level 1, the game runs ridiculously slow (~5 FPS) on my old Windows 8, I don't know what's causing the low framerate as the game isn't using much of my PC's resources.
The game seems to be more CPU-bound, though. I got terrible framerates (sub-20 fps on map 3) on an old system with a slow CPU but fast GPU (GTX 1070), and more playable framerates on a thin client with low-end Intel Graphics.
It also seemed to make no difference on an old gaming laptop with both Intel and NVidia GPU. I tried both, and got the same framerates.