The lack of parallelization for compilations within a single package doesn't matter too much when building go files, but does affect cgo compilation. I have a package with about 32k lines of (generated) C++. Compiling it serially takes ~10.5s. A small hack to cmd/go/build.go to compile C/C++/Obj-C files in parallel brings this down to ~3.7s.

The patch I hacked together to parallelize cgo compilation work is ~50 lines of deltas and is mostly contained in builder.cgo().

PS The compilation times above were measured using go 1.4.1 on Mac OS X, but this code hasn't changed at head and similar compilation times can be seen there.