Oh, well, we have different engines for rendering and for physics. We try to target a certain frame rate (e.g. 30 or 60hz). But currently, the physics and graphics are tied together: for one Gfx, we'll generate one simulation frame (physics).
I could add code to detect when a frame takes longer than it should have, and skip a graphics frame to make sure the physics run at the desired frame rate.
Say the user asks for 60hz, but in a situation, the graphics card is not good enough and takes too long to render the frame, I could detect that and render at 30hz for a few frames but keep the physics update at 60..