Source: I worked in VR 20 years ago for a defense contractor.
Sim Sickness is caused by a disconnect between what your eyes see and what your inner ear is telling you is happening. Your eyes are extremely sensitive to latency. If you snap your head quickly, even a small lag will cause a certain percentage of people to get nauseous. Having a fast and accurate motion tracking system is crucial, but you also need to have an extremely fast rendering engine and a headset capable of updating quickly as well. Motion prediction helps, also, but does not eliminate the problem. As does making sure your program doesn't require you to spin around a lot.
We can only put up with the horribly slow latencies on flat screen displays because they're not attached to our heads.