It's fast because of dependency analysis. They forgot about something called incremental compilation.
This is what they should have spent their 20% on: https://gcc.gnu.org/wiki/IncrementalCompiler
This is what they should have spent their 20% on: https://gcc.gnu.org/wiki/IncrementalCompiler
In practice compilation times are not a pain point for golang.
Also, Go does have incremental compilation at the package level.