main.go

package main

type Spannable interface {
	Len() int
}

type MyString struct {
	v string
}
type MyStringPointer struct {
	v *string
}

func (s *MyString) Len() int {
	return len(s.v)
}

func (p *MyStringPointer) Len() int {
	return len(*p.v)
}

func span(s Spannable) int {
	return s.Len()
}

func ss(s *MyString) int {
	return s.Len()
}

func l(s string) int {
	return len(s)
}

func cast(s interface{}) int {
	switch s := s.(type) {
	case string:
		return len(s)
	case *string:
		return len(*s)
	default:
		panic("unexpected type!")
	}
}

func main() {
}

main_test.go

package main

import "testing"

func BenchmarkInterfaceString(b *testing.B) {
	s := &MyString{"foo"}
	for i := 0; i < b.N; i++ {
		span(s)
	}
}

func BenchmarkTypeHintString(b *testing.B) {
	s := &MyString{"foo"}
	for i := 0; i < b.N; i++ {
		ss(s)
	}
}

func BenchmarkTypeSwitchString(b *testing.B) {
	s := &MyString{"foo"}
	for i := 0; i < b.N; i++ {
		cast(s.v)
	}
}

func BenchmarkInterfaceStringPointer(b *testing.B) {
	ss := "foo"
	s := &MyStringPointer{&ss}
	for i := 0; i < b.N; i++ {
		span(s)
	}
}

func BenchmarkTypeSwitchStringPointer(b *testing.B) {
	ss := "foo"
	s := &MyStringPointer{&ss}
	for i := 0; i < b.N; i++ {
		cast(s.v)
	}
}

func BenchmarkNative(b *testing.B) {
	s := "foo"
	for i := 0; i < b.N; i++ {
		l(s)
	}
}

result.txt

$ go test -bench .
testing: warning: no tests to run
PASS
BenchmarkInterfaceString        100000000               12.3 ns/op
BenchmarkTypeSwitchString       20000000               201 ns/op
BenchmarkInterfaceStringPointer 100000000               14.9 ns/op
BenchmarkTypeSwitchStringPointer        50000000                36.4 ns/op
BenchmarkNative 2000000000               1.49 ns/op