I've had the same problems. Sometimes, the renderer take upto three or four minutes, calculating the scene before rendering it on screen all at once. It's not very intuitive and has shown to be slower than XSI 1.5 due to the pre-processing. Bizarrely, I used to get this alot with hair but I managed (just twice) to get it rendering a complex hair scene almost straight away, updating the grids properly and everything. I can't get it working again.
I hate XSI as much as I love it. If my company had shelled out 11 grand on it, I'd be ready to sue by now.
KnickKnack