A lot of the tooling speed comes from a couple places:

1. The compiler optimizations being applied to code - Things like GADT's are able to be allocated very efficiently - Can generate code that doesn't box at runtime (smash it into a pointer is done as well) - Pointers are word-aligned

2. The OCaml runtime - No JIT - No warmup - Can emit native code

3. The HM type system - Typecheckers for HM are really simple [1]

[1] https://en.wikipedia.org/wiki/Hindley%E2%80%93Milner_type_sy...

This simplicity + the features of the compiler combine to make the OCaml compiler very fast, and very easy to write one (some undergrad CS classes do), and developer-time tenable to make both happen.