If you run tsc with benchmarking on, you'll find that almost all the time is taken up by the typechecker. Increasingly, TypeScript is adding control-flow-analys

If you run tsc with benchmarking on, you'll find that almost all the time is taken up by the typechecker. Increasingly, TypeScript is adding control-flow-analysis-based[1] and contextual type system features... Saying that it just 'removes' type annotations as a bit disingenuous, as processing the type constraints those annotations apply is one of the most time consuming tasks TS can perform! TypeScript is structural, too, so for every comparison it can need to get down to the nitty-gritty of comparing individual object members... Recursively. TS does heavy cacheing of not just types, but the relationships between types - just to avoid redoing any work, where possible. FYI, the 'helicoptering in' doesn't affect full-program compilation time because of the mentioned cacheing. Since the state of the world is unchanged during a build, the parsetree and type hierarchy are only built once. Additionally, you'd find that incremental builds (tsc --watch) are relatively snappy because of the heavy cacheing done and the tree reuse, and so has benefitted from the architecture improvements for service work. [1]https://github.com/Microsoft/TypeScript/pull/8010 https://github.com/Microsoft/TypeScript/pull/8010