Самый быстрый способ продублировать массив в JavaScript - слайс против цикла 'for'

function log(data) {
  document.getElementById("log").textContent += data + "\n";
}

benchmark = (() => {
  time_function = function(ms, f, num) {
    var z = 0;
    var t = new Date().getTime();
    for (z = 0;
      ((new Date().getTime() - t) < ms); z++)
      f(num);
    return (z)
  }

  function clone1(arr) {
    return arr.slice(0);
  }

  function clone2(arr) {
    return [...arr]
  }

  function clone3(arr) {
    return [].concat(arr);
  }

  Array.prototype.clone = function() {
    return this.map(e => Array.isArray(e) ? e.clone() : e);
  };

  function clone4(arr) {
    return arr.clone();
  }


  function benchmark() {
    function compare(a, b) {
      if (a[1] > b[1]) {
        return -1;
      }
      if (a[1] < b[1]) {
        return 1;
      }
      return 0;
    }

    funcs = [clone1, clone2, clone3, clone4];
    results = [];
    funcs.forEach((ff) => {
      console.log("Benchmarking: " + ff.name);
      var s = time_function(2500, ff, Array(1024));
      results.push([ff, s]);
      console.log("Score: " + s);

    })
    return results.sort(compare);
  }
  return benchmark;
})()
log("Starting benchmark...\n");
res = benchmark();

console.log("Winner: " + res[0][0].name + " !!!");
count = 1;
res.forEach((r) => {
  log((count++) + ". " + r[0].name + " score: " + Math.floor(10000 * r[1] / res[0][1]) / 100 + ((count == 2) ? "% *winner*" : "% speed of winner.") + " (" + Math.round(r[1] * 100) / 100 + ")");
});
log("\nWinner code:\n");
log(res[0][0].toString());

<textarea rows="50" cols="80" style="font-size: 16; resize:none; border: none;" id="log"></textarea>