String += within a loop issue

scale1

goos: linux
goarch: amd64
BenchmarkStringPlus-8         	   77480	    132547 ns/op	 1205014 B/op	       1 allocs/op
BenchmarkStringPlusFmt-8      	   63650	    124560 ns/op	 1203372 B/op	       3 allocs/op
BenchmarkStringBuilder-8      	51845487	        77.0 ns/op	     185 B/op	       0 allocs/op
BenchmarkStringBuilderFmt-8   	 7772652	       146 ns/op	     216 B/op	       1 allocs/op

scale10

goos: linux
goarch: amd64
BenchmarkStringPlus-8         	   10000	   1719950 ns/op	15540847 B/op	      10 allocs/op
BenchmarkStringPlusFmt-8      	   10000	   1854548 ns/op	18437905 B/op	      38 allocs/op
BenchmarkStringBuilder-8      	 5278312	       740 ns/op	    1826 B/op	       0 allocs/op
BenchmarkStringBuilderFmt-8   	  678554	      1819 ns/op	    2462 B/op	      10 allocs/op

scale100

goos: linux
goarch: amd64
BenchmarkStringPlus-8         	     763	  12973097 ns/op	118672419 B/op	     107 allocs/op
BenchmarkStringPlusFmt-8      	     654	  11944269 ns/op	117130386 B/op	     373 allocs/op
BenchmarkStringBuilder-8      	  508676	      4320 ns/op	   18949 B/op	       0 allocs/op
BenchmarkStringBuilderFmt-8   	   81718	     14272 ns/op	   20585 B/op	      99 allocs/op

scale1000

goos: linux
goarch: amd64
BenchmarkStringPlus-8         	     100	 170073279 ns/op	1554084884 B/op	    1079 allocs/op
BenchmarkStringPlusFmt-8      	     100	 173231980 ns/op	1744877880 B/op	    3837 allocs/op
BenchmarkStringBuilder-8      	   58435	     44040 ns/op	  164958 B/op	       0 allocs/op
BenchmarkStringBuilderFmt-8   	    7850	    142764 ns/op	  213962 B/op	     999 allocs/op

str_test.go

// Run benchmarks to showw the difference between string += and the string builder inside a loop. This also shows the
// minimal impact of fmt.Sprintf in comparison to using +=
package main

import (
	"fmt"
	"strings"
	"testing"
)

// this is adjusted for each run
const scale = 1

func BenchmarkStringPlus(b *testing.B) {
	var str string
	for i := 0; i < b.N; i++ {
		for n := 0; n < scale; n++ {
			str += "Benchmarking string and builder"
		}
	}
}

func BenchmarkStringPlusFmt(b *testing.B) {
	var str string
	for i := 0; i < b.N; i++ {
		for n := 0; n < scale; n++ {
			str += fmt.Sprintf("Benchmarking string and builder: %d", i)
		}
	}
}

func BenchmarkStringBuilder(b *testing.B) {
	var str strings.Builder
	for i := 0; i < b.N; i++ {
		for n := 0; n < scale; n++ {
			str.WriteString("Benchmarking string and builder")
		}
	}
}

func BenchmarkStringBuilderFmt(b *testing.B) {
	var str strings.Builder
	for i := 0; i < b.N; i++ {
		for n := 0; n < scale; n++ {
			fmt.Fprintf(&str, "Benchmarking string and builder: %d", i)
		}
	}
}