Now as we know Google backed down from requiring encryption on Lollipop devices. However, with my limited findings Samsung didn't have a thing to worry about.
I ran AndroBench 2 runs back to back on a clean boot with no background applications. I am posting up the fastest of each round. (The two runs were back and forth but within 3mb of each other or so)
Stock S6 64GB - 38GB free currently. The process took about 20 minutes on a full charge.
Before - No Encryption
After - Encrypted