test-loop-var-let.js

[1, 10, 100, 10000, 1000000, 100000000, 1000000000]
.forEach(size => {
  console.log('with', size)

  console.time('var');
  for (var i = 0; i < size; i++) {}
  console.timeEnd('var')

  console.time('let');
  for (let i = 0; i < size; i++) {}
  console.timeEnd('let')
});

Result with node v7.0.0:

$ node var-let-loop.js
with 1
var: 0.082ms
let: 0.014ms
with 10
var: 0.013ms
let: 0.002ms
with 100
var: 0.002ms
let: 0.002ms
with 10000
var: 0.028ms
let: 0.574ms
with 1000000
var: 0.599ms
let: 2.280ms
with 100000000
var: 57.668ms
let: 205.675ms
with 1000000000
var: 557.546ms
let: 2039.702ms