There's more...

Just for comparison, there is a benchmark code, which compares the performance of the built-in + operator, bytes.Buffer, and built-in copy:

1. Create a bench folder and file bench_test.go in it with the following content:

        package main

        import (
          "bytes"
          "testing"
        )

        const testString = "test"

        func BenchmarkConcat(b *testing.B) {
          var str string
          b.ResetTimer()
          for n := 0; n < b.N; n++ {
            str += testString
          }
          b.StopTimer()
        }

        func BenchmarkBuffer(b *testing.B) {
          var buffer bytes.Buffer

          b.ResetTimer()
          for n := 0; n < b.N; n++ {
            buffer.WriteString(testString)
          }
          b.StopTimer()
        }

        func BenchmarkCopy(b *testing.B) {
          bs := make([]byte, b.N)
          bl := 0
 
          b.ResetTimer()
          for n := 0; n < b.N; n++ {
            bl += copy(bs[bl:], testString)
          }
          b.StopTimer()
        }

1. See the results of the benchmark run: