Docker on Mac is actually running in a VM, since containers are a linux feature, not unix. You can't develop for linux servers like you can on a linux machine.
The GP is talking about how developing using docker on a Mac involves a separate VM running Linux, not the production deployment. If your application involves lots of files, then docker has to spend a bunch of time syncing files between the host OS and the Linux VM.
It's a pretty well known problem. If you're dealing with a few smallish files you won't notice it, but applications with lots of little files (Node, Python, etc) will have to sync all those files, and that can be pretty slow.
