I've never used neither Flow nor TypeScript, but there's an essential difference between the two - Flow is type inference engine, whereas TypeScript is a type checking engine. The latter is a fundamentally a much simpler problem (both semantically, and algorithmically) than the former. In addition, Flow picks a particularly nasty niche for type inference - OCaml itself uses type inference, it's fast (with quadratic blowup in edge cases) because OCaml's types are simple, whereas Flow IIRC tries to support both subtyping and some of JS's dynamicity, so I'm not surprised if it's slower.