Instead of creating a new XmlHttpRequest object for every request, this post provides code to pool these objects together and reuse them. This saves a great deal of processing times, particularly in IE.
Christian Heilmann does a tree menu without using loops. It demonstrates the flexibility and power of Event delegation - catching events at a higher level in the document. It drastically cuts down on the number of events you need to add to a document. One event handler per menu, rather than one per link in the menu.
Michael Mahemoff runs into a problem with Protypes $$ function when looking for class names. Its taking over 90 seconds when more than 100 matches are returned. The advice is: Avoid $$(".classname") on large DOMs.
Christian Heilmann demonstrates delegating events, and compares it to the main alternative of assigning event handlers to each individual node. Its a technique that's important for writing applications that can scale.
Apart from being an elegant debugger, FireBug offers the developer hooks that can be used from JavaScript, offering logging, assertions, performance measuring, and command line functions for inspection or traversing a document.
Ryan Campbell volume tests Prototype's each() Enumeration function and finds that although the code is more elegant than the for loops, the overhead is quite heavy, and the performance impact too high. The summary: If basic JavaScript will do the task, then use basic JavaScript