If you develop in Docker you probably, you probably mount a volume containing your source code from your host’s filesystem. If your host’s is running on osx (or windows), you might encounter slowness issues.
I myself stumbled upon this problem as I was running php tests, while a colleague had it with some python. I have had a bit of trouble figuring out why and how to fix it. Is it language related? Is it OS related? … I wrote this article to help you quickly understand the cause and tackle the problem.