Well, as you can see, only 4 buckets are working. Buckets are assigned one core/thread. They don't get assigned more cores. So only 4 cores/threads can be working on the render right now until it finishes. Which yes, does slow down the render. It would be nice if TG could cache a partial bucket, and then split that bucket up between idle cores, but that isn't functionality TG has right now. I think when that's possible, we'll also be able to suspend renders to cache files too, which would also be nice.