JSFiddle - React, Tailwind, and code Playground

JavaScript

var RUN_TIMES = 1000000;
var dt = new Date(2016, 0, 1);
dt.setHours(0, 0, 0, 0);
var dtStr1 = "";
var dtStr2 = "";

function pad(n){return n<10 ? '0'+n : n}


var now1 = new Date().getTime();

for (var i = 0; i < RUN_TIMES; i += 1) {
    dtStr1 = ("0" + (dt.getMonth() + 1)).slice(-2) + "/" + ("0" + dt.getDate()).slice(-2) + "/" + dt.getFullYear();
}

var t1 = new Date().getTime() - now1;

var now2 = new Date().getTime();

for (var j = 0; j < RUN_TIMES; j += 1) {
    dtStr2 = pad(dt.getMonth() + 1) + "/" + pad(dt.getDate()) + "/" + pad(dt.getFullYear());
}

var t2 = new Date().getTime() - now2;

alert("using slice():\n" + dtStr1 + "\t" + t1 / 1000 + "\n\n" + "using pad():\n" + dtStr2 + "\t" + t2 / 1000);