Code optimization is both an art and a science. This means that there is no deterministic way to help you optimize your Go code, or any other code in any programming language, and that you should use your brain and try many things if you want to make your code run faster.
You should make sure that you optimize code that does not contain any bugs, because there is no point in optimizing a bug. If you have any bugs in your program, you should debug it first.
If you are really into code optimization, you might want to read Compilers: Principles, Techniques, and Tools by Alfred V. Aho, Monica S. Lam, Ravi Sethi, and Jeffrey D. Ullman (Pearson Education Limited, 2014), which focuses on compiler construction. Additionally, all volumes in The Art of Computer Programming series by Donald Knuth (Addison-Wesley Professional, 1998) are great resources for all aspects...
