Do not preconvert to string as it apparently has minimal performance benefit.

src/array.js

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 694 matching lines @@
     }
     x = ToString(x);
     y = ToString(y);
     if (x == y) return 0;
     else return x < y ? -1 : 1;
   };
 
   function InsertionSort(a, from, to) {
     for (var i = from + 1; i < to; i++) {
       var element = a[i];
-      var key = %_IsSmi(element) ? element : ToString(element);
       for (var j = i - 1; j >= from; j--) {
         var tmp = a[j];
-        var order = Compare(tmp, key);
+        var order = Compare(tmp, element);
         if (order > 0) {
           a[j + 1] = tmp;
         } else {
           break;
         }
       }
       a[j + 1] = element;
     }
   }
 
   function QuickSort(a, from, to) {
     // Insertion sort is faster for short arrays.
     if (to - from <= 22) {
       InsertionSort(a, from, to);
       return;
     }
     var pivot_index = $floor($random() * (to - from)) + from;
     var pivot = a[pivot_index];
-    // Pre-convert the element to a string for comparison if we know
-    // it will happen on each compare anyway.
-    var pivot_key = %_IsSmi(pivot) ? pivot : ToString(pivot);
     // Issue 95: Keep the pivot element out of the comparisons to avoid
     // infinite recursion if comparefn(pivot, pivot) != 0.
     a[pivot_index] = a[from];
     a[from] = pivot;
     var low_end = from;   // Upper bound of the elements lower than pivot.
     var high_start = to;  // Lower bound of the elements greater than pivot.
     // From low_end to i are elements equal to pivot.
     // From i to high_start are elements that haven't been compared yet.
     for (var i = from + 1; i < high_start; ) {
       var element = a[i];
-      var order = Compare(element, pivot_key);
+      var order = Compare(element, pivot);
       if (order < 0) {
         a[i] = a[low_end];
         a[low_end] = element;
         i++;
         low_end++;
       } else if (order > 0) {
         high_start--;
         a[i] = a[high_start];
         a[high_start] = element;
       } else {  // order == 0
@@ skipping 420 matching lines @@
     "lastIndexOf", getFunction("lastIndexOf", ArrayLastIndexOf, 1),
     "reduce", getFunction("reduce", ArrayReduce, 1),
     "reduceRight", getFunction("reduceRight", ArrayReduceRight, 1)
   ));
 
   %FinishArrayPrototypeSetup($Array.prototype);
 }
 
 
 SetupArray();