It used to be if you had for instance mutiple tracks and the top track was a DV-AVI file with 100% opacity and no filters, no rendering was required. Now it seems that if you have anything on the lower tracks, even if completely hidden by the top layer, rendering is always required and transcoding takes longer - sometimes considerably depending on what's on the lower layers though not seen. I actually have to completely remove sections of clips on the lower tracks to speed up transcoding. Just more unnecessary work and time.
Forgive me if this has been discussed before, but I can not seem to find this precise issue among the slow rendering discussions.