It seems that, for whatever reason, the linux build isn't handling render textures properly. That's going to make the game incredibly jerky and so forth at 20fps or less. Most likely the fps being that low is the cause of the squid acting crazy. I'd need to shift a few things to fix that, but if I do that the game is going to start playing slower, perhaps at 3/4 speed or so in your case, and that's not exactly going to be an ideal experience either...
Render textures seem to be the culprit here, though, unless you have a GPU with a incredibly low pixel fill rate. But judging by your numbers on the other games, I think we can rule that out; the only difference here is render textures that actually result in lower draw calls than the other games, which is usually the big slowdown. On most machines that get 30fps on Valley 2, they get closer to 60fps on Shattered Haven...