Largely, yes. It is an actual map with a skybox, but the models you see in the title screen aren't actually Actor entities.
I'm actually doing a port of the Actor class into C++. The only best guess I have left is that GDScript (Godot's built-in scripting language) just can't pull off the number of physics requests that I'm asking of it no matter how much I try to optimize it.
It's just very weird how dramatically inconsistent it is across higher end PCs, but GDNative/C++ is significantly faster than GDScript so there should be improvements no matter what.
I don't have an ETA on the C++ update currently. Many apologies. Just trying to do this right.