Neither of those check types. It’s easy to be fast when all you’re doing is filtering out type-bytes (and perhaps some transpiling). That’s nothing compared to actually type checking, which requires solving complex satisfiability problems across the entire codebase.

My understanding was that TypeScript would be more parellisable than Flow, which itself is plenty parallelisable: https://arxiv.org/pdf/1708.08021.pdf

Call me crazy, but I prefer single threaded. It limits the potential impact on the rest of the system, and all cloud payment is by the core•hour anyways.

Do they check types?

Sorry, my previous comment wasn't very clear. Of course, faster languages produce faster results. But the language alone isn't enough in this case.