GPU encoding is fast, but usually it produces poorer quality results because it avoids trying paths that are hard to do quickly on the GPU.

Driveby 2c as a video industry person: don't retranscode your media unless you've got them in a really space inefficient codec and you're seriously hurting for space. You'll burn a lot of power retranscoding, are you actually saving useful $$$ of storage in exchange for that spend? Storage is cheap, and there's always a better codec coming along you could retranscode into and save some more space. It's a vicious cycle: each generation has to encode the artifacts from the previous generations.