Feature/fix perf

/to @ritch 
/to @bajtos 

Here are a few patterns I use:

1. Convert array.forEach(fn) to for(var k = 0; k < keys.length; k++) {...};
See: http://jsperf.com/fast-array-foreach

2. Avoid Object.defineProperty()
See: http://jsperf.com/object-defineproperty-vs-definegetter-vs-normal/21

3. Avoid array.indexOf() 
The penalty is bigger than I thought: http://jsperf.com/for-vs-foreach/165

Looks like SpiderMonkey has a much better optimizer for different loop styles.