[citation][nom]sykozis[/nom]Why worry so much about JavaScript performance? Java performance is horrid in comparison.... Find ways to accelerate Java...[/citation]
How can you compare the two? JavaScript is a staple of modern websites - a vast amount of interactive content is built using it and its usage is ever increasing. Even online office suites (Docs for example) are JS apps.
Speeding up JS execution has become a deciding factor in speed - all browsers are great at HTML (okay, IE and Safari lag in HTML 5, but are soon going to make that right) and the speed penalty of it is far lower. Same thing for CSS. Flash is independent from the browser itself. So what's left? JS.
Java applets are used frequently on the web, but not as often as they used to be and there rarely are hundreds of them on a website (thank god). There's room for improvement, but comparing Java to JS is like comparing mangos to cereal crops - you need both, it's clear which one you need the most. Oh, and Java is far more browser independent (a bit like flash - provided by an external plugin. The only thing that differs greatly between browsers is the stability of the plugin, not its speed).