I think people often underestimate how much the TypeScript team does to make TypeScript fast and efficient. They have some of the best people in the field working on the type checker

Yes, because it's one of the slowest compilers in the world, and its job is to compile from JavaScript to JavaScript. There are other languages with complex type systems, where the compilers have to do a lot of other additional work on top of typechecking, and are still way faster than TS.

And TypeScript is always slow, not only when you do abuse its type system and require it to do complex inference - I mostly use it for "this is a number" and "this is a object with the following 4 fields" and it's still by far the slowest component in my builds usually

Indeed. I haven't checked the implementation, but I suspect that "don't spin up a separate process for every file" is an idea that has probably already occurred to them :D