You don't need to parse the environment from scratch 60 times every second. As long as you get head tracking right you can just focus on what's moving and assume it will keep moving the same way. Further the demos all seem to be around a fairly static environmenment. Remeber you don't need to render the background so a simplified internal model works fine with AR.

If it works near a closeline blowing in the wind that's really hard and impressive.