That's probably it, then. Especially if it's rendering anything at a decent distance. More complex Unity games with long draw distances tend to be the hardest for my machine to deal with. It took a ton of tinkering to get (for example) Edge of Eternity to run decently for me because the scenery in the world is so far off to create a sense of scale. I can see pretty far into the distance in Witness, so that might be the culprit. I'd suck it up at like 15-20 FPS since I really want to play it, but can't even squeak more than about 3 FPS out of it.