I wrote physics simulation code some time ago. The build was fast, but since the simulation speed was about 10ns/day the tests would take several hours to run.

I wrote physics simulation code some time ago. The build was fast, but since the simulation speed was about 10ns/day the tests would take several hours to run. And the only way to know if your change fixed something is if the tests passed. So it took several hours to get feedback for a change. But it also took about that long to come up with a new idea to try. Eventually I had a pipeline running where I could fire off a few ideas throughout the day, keep track of the change in the build, and iterate that way.