Try it out:
http://jsbench.appspot.com/
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If you aren't sure how to get started, check out some of my experiments.
For instance, did you know that using the Boolean() constructor is perhaps the slowest way to create a boolean? In some cases it can be as much as an order of magnitude slower than alternate methods.
http://jsbench.appspot.com/#suite=agdqc2JlbmNocgwLEgVTdWl0ZRiaAgw
Also, if you've been using a for..in loops when they aren't necessary, your code is probably running more than ten times slower than it has to if you'd used a typical for-loop and cached the length.
http://jsbench.appspot.com/#suite=agdqc2JlbmNocgsLEgVTdWl0ZRgNDA
Oh, and the last bit for now is that you better not even think of omitting the "var" statement when creating a new variable. Doing so could be slowing you down by two orders of magnitude!
http://jsbench.appspot.com/#suite=agdqc2JlbmNocgwLEgVTdWl0ZRjqAgw
Look for some more specific, in depth studies to come. What examples can you come up with?