How to get reliable physics simulation?

hi,

I'm running a number of physics simulation in parallel as part of a genetic algorithm experiment.
to keep the training time short, I want the simulations to run as fast as possible. whereas, when replaying the simulation, I want them to run in real-time.
for that reason, and for making the simulation as deterministic as possible, I am disabling Physics.autoSimulation, and go through the simulation in fixed time steps:

Code (CSharp):

1. IEnumerator simulate() {
2.     Physics.autoSimulation = false;
3.     // Application.targetFrameRate = 50;
4.  
5.     float maxDuration = 10f;
6.     float timeStep = Time.fixedDeltaTime;
7.     float t = 0f;
8.  
9.     while (true) {
10.         if (t > maxDuration) {
11.             break;
12.         }
13.  
14.         Physics.Simulate(timeStep);
15.         t += timeStep;
16.  
17.         // a)
18.         // expectation: runs as fast as possible
19.         // reality: physics don't seem to get applied at all
20.         continue;
21.  
22.         // b)
23.         // expectation: runs as fast as possible
24.         // reality: speed seems to be real-time, more or less
25.         yield return new WaitForSecondsRealtime(0f);
26.  
27.         // c)
28.         // expectation: proper physics (same as b)), but slowed down
29.         // reality: slow indeed, but collisions don't work reliably, for instance
30.         yield return new WaitForSecondsRealtime(0.5f);
31.     }
32.  
33.     yield break;
34. }

the problem I am encountering though is that despite calling Physics.Simulate(timeStep) with constant values, the presence or absence of WaitForSecondsRealtime in the loop seems to have an influence on the physics simulation.

I'm at a loss here. the whole point of controlling physics yourself is that it is independent of the "wall time" (the real time passed). — how do I properly do this?

thanks a lot in advance!