[microtonal]

Local type inference is not necessarily expensive. Type inference is abundantly used in Go, and yet it's probably one of the fastest compilers.

There are also languages that do Hindley–Milner type inference (ML, Haskell) with good compile times.

[spullara]

I am sure that you can have much cheaper type inference. However, if you turn on the compiler profiler for Scala, you can see that most of the time is spent in type inference. They don't do HM.