The funny thing about "moving quickly in dynamic languages" is that it doesn't last. Refactoring code in a dynamic language gets difficult fast and is an entirely hopeless endeavour without a huge test suite (you obviously need tests on every level, because you can't refactor an interface and its test at the same time and still think it's working as intended).

Typed code may be slower and more cumbersome (to some) to write in the first place, but is usually much easier to maintain in my experience.