From the 1.5 release notes<p>Builds in Go 1.5 will be slower by a factor of about two. The automatic translation of the compiler and linker from C to Go resulted in unidiomatic Go code that performs poorly compared to well-written Go. Analysis tools and refactoring helped to improve the code, but much remains to be done. Further profiling and optimization will continue in Go 1.6 and future releases. For more details, see these slides and associated video.