https://gopher.tips/2018/07/11/is-setting-slice-capacity-an-over-optimization

main.go

package main

import "fmt"

func main() {
	fmt.Printf("%v\n", WithoutCap())
	fmt.Printf("%v\n", WithCap())
}

func WithoutCap() []int {
	slice := make([]int, 0)
	for i := 0; i <= 100000; i++ {
		slice = append(slice, i)
	}
	return slice
}

func WithCap() []int {
	slice := make([]int, 0, 100000)
	for i := 0; i <= 100000; i++ {
		slice = append(slice, i)
	}
	return slice
}

main_test.go

package main

import "testing"

func BenchmarkWithoutCap(b *testing.B) {
	for n := 0; n < b.N; n++ {
		WithoutCap()
	}
}

func BenchmarkWithCap(b *testing.B) {
	for n := 0; n < b.N; n++ {
		WithCap()
	}
}