I just realized that there is a bug in applying forces/torques to bodies. When phent_addcentralforce() or a similar function is called, the force will only be applied for one physics frame (usually running at 70Hz, i.e. force will last for 1/70th of a second). If the rendering framerate is less than 70Hz additional physics frames will be executed for each rendering frame. E.g. if running at 35 frames render speed for every "wait(1)" two physics frames will be calculated, the first one has user-specified forces/torques, the second one only has gravity and constraint forces.
On slower systems this means that forces appear to have less of an impact on a body's motion. I will change this behavior so that user forces are applied throughout the entire time of a "wait()" statement. Please note that this is likely to break existing code if you explicitely set forces/torques in your script.