The majority of the performance difference between strings concat and builder in your example is explained by memory allocation. Every loop of concat will resul

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 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