This is a great talk by @KevlinHenney
When people write tests, they write less code.
Take a step back and try to look at the code from your colleagues perspective (at 24:00):
If I think it's difficult to use, what do the others think?
He says about the question if tests slow you down (at 1:33:01):
Yes, tests are your breaks. They will allow you to go faster, for the very reason they slow you down.