iPhone OS 2.0 twice as fast at JavaScript
http://www.iphoneunity.com/?p=212

The current iPhone’s relatively poor JavaScript has been a point of
lament for users experiencing slow rendering of AJAX pages and a
virtual inability to use JavaScript-heavy, Web-based games. With the
latest builds of iPhone OS 2.0, however, Apple has dramatically
boosted JavaScript performance. While the new MobileSafari’s
performance still doesn’t rival that of contemporary desktop browsers,
the increased JavaScript quickness will aid significantly in the
rendering of some Webmail interfaces and other interactive sites.