var t0 = performance.now()
doSomething() // <---- measured code goes between t0 and t1
var t1 = performance.now()
console.log("Call to doSomething took " + (t1 - t0) + " milliseconds.")
или в лог (строки в time и timeEnd должны быть одинаковы):
console.time('doSomething')
doSomething() // <---- The function you're measuring time for
console.timeEnd('doSomething')