Docker does a lot of mumbo jumbo with the disk, I'm guessing that's why. Start a container with a host folder mapped in and do something moderately IO-intensive, like clone a large-ish git repo, and you will notice it's a lot slower than bare metal. I'm guessing this is because docker uses union file systems heavily, and that adds IO overhead. You'd probably get more or less equivalent performance to bare metal if all you did was set up a simpler form of container which just configured some namespaces.

TL;DR: Containers are fast, docker specifically does extra stuff which is slow.