Have you tried direct restore from backup files? It should only do a partial database rebuild
It looks like they are trying to implement multithreading right now, not sure if this will help with the database rebuilding.
Larger block sizes make the database a little easier to manage, with some tradeoffs.
Unfortunately this setting can’t be changed after the first backup. It’d be great to see variable chunk sizes (e.g. using the popular buzhash algorithm)