At 26:40 "I've been toying with this hypothesis that fast tests lead to good code. It might be a bi-directional relationship. If the tests have to be slow, it might be because there are issues in the code. If the tests are fast, it might be because the code is well factored. (other guy) A lot of times, if your tests are slow it's because you have a lot of coupling between your classes, so they're difficult to test in isolation, so you involve all these other parts, and that makes it slow."
"This is a large part of the claim that isolated tests drive better design. Because an isolated test must set up all collaborators as mocks, the only way to reduce the mock complexity is to reduce the depth of collaboration"