Let’s say that you want to know if EncodeToString is faster than fmt.Sprintf : you will need to compare the speed of this method

func Md5Encode(str string) string {
	md5HashInBytes := md5.Sum([]byte(str))
	md5HashInString := hex.EncodeToString(md5HashInBytes[:])
	return md5HashInString
}

with this other one

func Md5EncodeFmt(str string) string {
	md5HashInBytes := md5.Sum([]byte(str))
	md5HashInString := fmt.Sprintf("%x", md5HashInBytes)
	return md5HashInString
}

Go provides benchmarking features in the testing package which is pretty usefull :

func BenchmarkMd5EncodeFmt(b *testing.B) {
	// run the md5Encode function b.N times
	for n := 0; n < b.N; n++ {
		Md5EncodeFmt("aldfhasdl la fasdfeo8ekldjh asdkj fh lksdjfhwoieuxnroiAUN;laiDJ;ANIfub;OEIRBUF;OEfuN;ALFJ;AL")
	}
}

func BenchmarkMd5Encode(b *testing.B) {
	// run the md5Encode function b.N times
	for n := 0; n < b.N; n++ {
		Md5Encode("aldfhasdl la fasdfeo8ekldjh asdkj fh lksdjfhwoieuxnroiAUN;laiDJ;ANIfub;OEIRBUF;OEfuN;ALFJ;AL")
	}
}

Run

$ go test -bench=.
goos: linux
goarch: amd64
BenchmarkMd5EncodeFmt-8   	 1894791	       625 ns/op
BenchmarkMd5Encode-8      	 3068509	       363 ns/op
PASS
ok  	_/home/paul/LazyInit/bench	3.342s

Run 3 times the benchmarks :

$ go test -count 3 -bench=. 
goos: linux
goarch: amd64
BenchmarkMd5EncodeFmt-8   	 1882105	       627 ns/op
BenchmarkMd5EncodeFmt-8   	 1918942	       624 ns/op
BenchmarkMd5EncodeFmt-8   	 1902894	       625 ns/op
BenchmarkMd5Encode-8      	 3139585	       386 ns/op
BenchmarkMd5Encode-8      	 2937154	       397 ns/op
BenchmarkMd5Encode-8      	 3009801	       380 ns/op
PASS
ok  	_/home/paul/LazyInit/bench	10.217s

EncodeToString() makes your method almost twice faster !

In the case you need to isolate the code that will be measured from some init/fixture code you can use ResetTimer() and StopTimer() to accurately isolate what you want to measure :

func BenchmarkMd5EncodeFmt(b *testing.B) {
	// Some init code
	initRandomBenchStrings()
	b.ResetTimer()
	for n := 0; n < b.N; n++ {
		Md5EncodeFmt(getRandomstrings())
	}
	b.StopTimer()
	// some final code
}