Rendering faster the transparent areas in Cycles So, is there anyway to render faster the transparent areas of a 3D animation?
For example: my background is transparent and most of the image will be transparent, but for some reason Cycles takes minutes to render these transparent areas.
Branched path tracing is significantly faster on empty background areas since it limits initial samples and instead gathers multiple subsamples: What is branched path tracing and how is it useful?

Some effects such as depth of field or motion blur may need a lot of initial samples anyway though, so branched path may not always be faster. Additionally, it consumes quite a lot of extra memory when running on GPU, so it can easily put a scene over the edge of fitting in VRAM or not.