JSFiddle - React, Tailwind, and code Playground

JavaScript

var arr1 = [],
  arr2 = [],
  obj1 = {},
  obj2 = {}; // get max array length

for (i = 0; i <= 10000; i++) {
  arr1.push(i);
  arr2.push(i);
}

for (i = 10001; i <= 20000; i++) {
  arr2.push(i);
}


function oneLoop() {
  obj1 = {};
  obj2 = {};
  var length = Math.max(arr1.length, arr2.length);

  for (var i = 0; i < length; i++) { // loop max array length

    if (i < arr1.length) { // do check for arr1
      obj1[i] = arr1[i];
    }

    if (i < arr2.length) { // do check for arr2
      obj2[i] = arr2[i];
    }
  }
}

function twoLoops() {
  obj1 = {};
  obj2 = {};

  for (var i = 0; i < arr1.length; i++) { 
      obj1[i] = arr1[i];
  }
  for (var i = 0; i < arr2.length; i++) { 
      obj2[i] = arr2[i];
  }
}

var t0 = performance.now();
oneLoop();
var t1 = performance.now();
document.body.appendChild(document.createTextNode("Call to oneLoop took " + (t1 - t0) + " milliseconds."));
document.body.appendChild(document.createElement('br'));

var t2 = performance.now();
twoLoops();
var t3 = performance.now();
document.body.appendChild(document.createTextNode("Call to twoLoops took " + (t2 - t1) + " milliseconds."));
document.body.appendChild(document.createElement('br'));