Douglas Crockford's presentation on Advanced JavaScript. He covers topics such as inheritance, modules, debugging, efficiency and JSON.
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.
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