Does Kdenlive support prerendering? Really, there are two parts to this:
1) generating video files of transitions (invalidated when the transition's configuration changes)
2) using existing video data when the export format matches the input

A convenient #3 would be:
3) support prerendering full video clips in alternative formats

What this would do is allow someone to render a large project once, and have a very fast export, only requiring actual rendering and encoding for the small parts they make changes to.