jit.phys.world global simulation time step / speed change

Hi, I'm new to jit physics and managed a first mass–spring model, but would like to change the speed of the movements dynamically (but keep the same frame rate). I tried fixedtimestep, but this is not changing the simulation time step.

Note that I'm running jit.phys.world from a metro, because I don't need any rendering, just the physics. Slowing the metro slows the movement, of course, but makes it choppy.

Anything that I missed?