> There are a lot of things that make Go slower to write than Ruby, but mandatory typing isn't one of them. Rather, Ruby's total lack of typing makes it harder to write: you effectively end up having to write unit tests just to catch typos.

No, you don't. You need unit tests to verify behavior (values) whether or not you have static typing (except for output types with only zero or one values). Now, it's true, that having such tests also verifies, at no extra charge, things that Go’s type system would verify, but without the additional effort of type annotations. But there's no added cost there, it's a net savings.