Putting Performance In Perspective

The internet is rife with all sorts of advice about performance, and jQuery and JavaScript performance in particular. Unfotunately, a lot of this filters down into a context-free echo chamber that has mixed-results in end-developer code. For instance, you'll see people who think nothing of nesting several AJAX calls, but then they'll end up doing a manual for loop over jQuery selections in in those nested callbacks because they "heard $.each was slow." We should breathe some sanity into this discussion and provide context for our other performance tips by explaining basic things like

• importance of profiling
• optimizing the things that will give you the biggest payoff/that have room for optimation (don't optimize the thing taking 1% of the time)
• balancing performance with maintainability/readability
• avoiding premature optimization, but also, what to prematurely optimize, e.g., caching jQuery collections is not only better because it's faster, but because it improves readability and code structure
• library problems are likely different than your problems. in other words, if your app is slow, it's probably not because of scope chain traversal
• if the only thing slow about your app is the overhead of $.each, have a Diet Coke