The Go compiler is already much faster than your typical C and C++ compiler. Now the compilation step, which is done on those projects via make -j4, can be done natively in the go compiler within modules by itself, compiling functions in one module in parallel. Modules itself would be already parallelizable by make -j4, but since go build is its own tool independent of make, they re-invented it by themselves. It's significantly faster than make already, but comes with maintenance costs. go build is much stricter than traditional dependency systems.

It's not really significant at all. Compile-times of bigger modules will get a bit faster, such as e.g. with zapcc, which got C++ compile times 50% faster by caching.