Actually, last year Twitter launched a JavaScript-heavy rewrite that was much slower to load and interact with than it was previously. It seems to have improved since then, but at the time it was fairly tiresome to use.
It's a bit bare-bones, but IMO totally makes up for it in raw plain-HTML speed.
I use twirssi myself, but a common complaint I've heard of the twitter site istself is that its auto-refresh will destroy work-in-progress tweets.
(The 140 char limit can lead to serious time spent crafting responses without resorting 2dis typ o stf)