Yes on the backend AppGameKit uses threads for physics and HTTP ETC just as we can use threads in our C++ or plugin code, Box2D does not draw, it tells the sprite class where to draw after its done its calculations in the thread so its safe to use in a thread, I would suspect much of the 3D stuff is in a thread or 2 as well
Problem is I cant just thread every agk function inside a plugin, give me a problem or situation and I can give you a threaded solution but something like this would need to be hand made to suit the project needs, there is no, and will never be no all-in-one generic solution.
Compute, yeah I never understood that one TBH, I can achieve the similar results piping to a plugin thread so I never gave it much of a look but looks like it pipes the logic out to the GPU, not exactly multithreading but it takes the strain off the CPU so it could be a solution, just need to learn HLSL, either way there is a language to learn I'd just go with C++