I don't think that a calculation of 100ms should need the double of time of a calculation of 50s, because the time component is simply passed to physical calculations which don't iterate through time, but use the elapsed time as a constant for movement. In special cases where objects move very fast and could pass through other objects in one step maybe rays are used to calc the movement and collision.

So every calculation in your example should need nearly the same time to process no matter if 10 ms or 10 seconds elapsed.

I think thats the way it is done in games, don't know how it is done in physical simulations.