The majority of the performance difference between strings concat and builder in your example is explained by memory allocation. Every loop of concat will result in a new allocation, while the builder - which uses []bytes internally - will only allocate when length equals capacity, and the newly allocated slice will be approx. twice the capacity of the old slice (see: https://golang.org/src/strings/builder.go?#L62).

Therefore, 500,000 rounds of concat is about 500,000 allocations, while 200,000,000 rounds of builder is ~ 27.5 allocations (=log2(200000000)).

I would suggest a different benchmark to approximate real world usage:

    func BenchmarkConcatString(b *testing.B) {
        for n := 0; n < b.N; n++ {
            var str string
            str += "x"
            str += "y"
            str += "z"
        }
    }

    func BenchmarkConcatBuilder(b *testing.B) {
        for n := 0; n < b.N; n++ {
            var builder strings.Builder
            builder.WriteString("x")
            builder.WriteString("y")
            builder.WriteString("z")
            builder.String()
        }
    }

Which still shows a significant performance advantage for using builder (-40% ns/op):

    BenchmarkConcatString-4     20000000            93.5 ns/op
    BenchmarkConcatBuilder-4    30000000            54.6 ns/op