see console

<script>

    var len = 500000;


    var oj = {};
    for (var i = 0; i < len; i++) {
        oj["b" + i] = [1, 2, 3, 4, 5];
    }

    var ary = [];
    for (var i = 0; i < len; i++) {
        ary.push(["a" + i, 1, 2, 3, 4, 5]);
    }


    /*
     * TEST for Object
     */

    var dmy = '';
    console.time("oj====================");
    for (var i in oj) {
      dmy =  i + oj[i][0]; 
    }
    console.timeEnd("oj====================");

    console.log(dmy);

    /*
     * TEST for Array
     */

    var dmy = '';
    console.time("ary====================");
    for (var i = 0; i < len; i++) {
      dmy = ary[i][0] + ary[i][1]; 
    }
    console.timeEnd("ary====================");

    console.log(dmy);

</script>

oj====================: 566.000ms test.htm:27
b4999991 test.htm:29
ary====================: 39.000ms test.htm:40
a4999991 test.htm:42