this might depend on many things. Is that the GPU load even when unfocused? (The UI is gone)
I have set it up so that the app unloads all the gpu-heavy UI-Stuff when unfocused.
Additionally it could depend on the imported models, since they are essentially always loaded, so they can be quickly spawned for the throw. They are not rendered when inactive, but they live in memory.
As someone mentioned, the default FPS is at 120 FPS. In the experimental version the graphics settings were fixed and should allow for a change of the framerate properly