Whenever you get into a situation where SpeedTree needs a bit of extra time to calculate, I'm always presented with corrupt or old content out of the Frame Buffer. This is ugly and not in the least a bit disconcerting every time it happens (I always think SpeedTree is crashing). It would be good form to not let this happen, and instead, not have SpeedTree update the viewport until the calculations are done and a new image can be rendered.