If you minimize Universe Sandbox it stops drawing the screen, but still draws the screen even if another window is completely covering it (outside of custom code, I'm not sure there's any way of doing this, but I might be wrong).
When Universe Sandbox loses focus it sets its process priority to "Below Normal" and when you switch back, it resumes at "Normal" priority. (These are the same as the process priorities in Task Manager)
Either way... Drawing the screen doesn't take as much time as you might think once you have over 100 objects with mass, there the gravity calculation become what take the longest.
Saturn and its moons (with about 60 objects spends about 1/3 of the time drawing the screen on my machine).
So even without drawing the screen there's no way to get a 10x speed increase.