As with everything, take "almost no cost" with a grain of salt. There sure is situations where docker can be slow compared to bare metal, even on linux.
Most things are running basically the same when using containers, but not everything. Take networking as an example: That's usually not working as it would on bare metal and usually result in a significant performance loss.
Just use overlayfs and your performance issues are history. Works perfectly on Ubuntu 16.04 and I hope nobody needs to use significantly older systems.
Hah, good one.