If you want to optimise, try different encoders (sounds like you've already done some of this) and lots of different settings - it'll involve a lot of tuning if you want to figure out the right balance for your particular media between quality/speed/size, while also making sure that your machine hurts as much as possible.
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.