github.com/json-iterator/go is currently one of the fastest libs available to process json.
Besides, it is a simple drop-in replacement for the standard lib, without much code change.

I am proposing PR #357 to validate the concept and expose benchmarks on signing and parsing methods.

NOTE: due to some changes in the order of marshalled claims, testable examples verifying the value of the signed string have been adapted to check for decoded claims instead.