Strange Object vs. Array benchmark result under --debug-brk flag

Question

I want to decide what is best structure in javascript for small tree nodes. Each node have key, value and references to the parent and child nodes (if exists). So I wrote benchmark for testing performance of the nodes creation:

var Benchmark = require('benchmark');
var suite = new Benchmark.Suite();

suite
.add('Object node creation', function() {
    var parent = {
        key:    'some_key',
        value:  1234123412,
        parent: null,
        left:   null,
        right:  null
    };

    var left = {
        key: 'left child',
        value: 'asdfoasjdfpasdjf',
        parent: null,
        left:   null,
        right:  null
    };

    var right = {
        key:   'right child',
        value: 'qwerqwerqwerqwwq',
        parent: null,
        left:   null,
        right:  null
    };

    parent.left  = left;
    parent.right = right;
    left.parent  = parent;
    right.parent = parent;
})
.add('Array node creation', function() {
    var parent = ['some_key',    1234123412,         null, null, null];
    var left   = ['left_child',  'asdfoasjdfpasdjf', null, null, null];
    var right  = ['right_child', 'qwerqwerqwerqwwq', null, null, null];

    parent[3] = left;
    parent[4] = right;
    left[2]   = parent;
    right[2]  = parent;
})
.on('complete', function() {
    console.log(this[0].toString());
    console.log(this[1].toString());

    console.log('Fastest is ' + this.filter('fastest').pluck('name'));
})
.run();

So I run the test:

node benchmark.js

and have the following results:

Object node creation x 30,613,857 ops/sec ±3.11% (82 runs sampled)
Array node creation x 15,133,139 ops/sec ±1.12% (94 runs sampled)
Fastest is Object node creation

And it's OK.

But then I run benchmark with --debug-brk:

 node --debug-brk benchmark.js

and get the following results:

Object node creation x 4,842,367 ops/sec ±2.81% (90 runs sampled)
Array node creation x 10,906,219 ops/sec ±2.36% (90 runs sampled)
Fastest is Array node creation

As you see: performance of object creation was decreased drastically. Maybe someone can explain me why it so and what is happened.