I haven't found any large scale analyses but here's an example of a simple type annotation halving compilation time: https://stackoverflow.com/questions/36624273/investigating-l...

The Typescript compiler performance wiki might also be of interest: https://github.com/microsoft/TypeScript/wiki/Performance

I haven’t personally benchmarked it, but I know it’s available as an eslint rule (all exported functions must have declared return types).

This makes sense for semantic reasons though. If you accidentally return a `number` when you returned a `string` in some earlier branch, Typescript will happily unify them and now your function silently returns `string | number`. The linting rule prevents this.

I always thought this is because some people prefer this style.