I have a pretty complex scene. When I render it at about 1440 x700 it renders very quickly ,about 5 mins. But when I crank up the resolution to something like 4500 x 2187 it starts pretty quickly, but after a few buckets finishing it gets very very very slow. And after about 2 hours the buckets just pretty much stop rendering completely. If I stop it, and start it back up, it goes nice and fast again, but then slows again.
Has anyone ever seen? Its starting to drive me crazy.