let results = [];

let sum = 0;

localStorage.setItem('foo', 'foo bar baz foo bar baz foo bar baz foo');

​

for(let i =0; i < 1000000; i++){

    let a = performance.now();

    localStorage.getItem('foo');

  let result = performance.now()  -a;

  sum += result;

  results.push(result);

}

​

console.log(`Item found: ${sum/results.length}ms per call`);

​

results = [];

sum = 0;

for(let i =0; i < 1000000; i++){

    let a = performance.now();

    localStorage.getItem('bar');

  let result = performance.now()  -a;

  sum += result;

  results.push(result);

}

​

console.log(`Item not found: ${sum/results.length}ms per call`);