Django Tests run faster with no internet connection

I have a django test suite that builds a DB from a 400 line fixture file. It runs unfortunately slow. Several seconds per test.

I was on the train yesterday developing without internet access, with my wifi turned off, and I noticed my tests ran literally 10x faster without internet. And they are definitely running correctly.

Everything is local, it all runs fine without an internet connection. The tests themselves do not hit any APIs or make any other connections, so it seems it must be something else.