Except that the complaints about Twitter's new interface are all related to Javascript performance, and have nothing to do with CSS. Their toolkit release was all about CSS.

Lots of CSS written in the wrong way can actually be pretty processor-heavy.

Assuming you mean the twitter bootstrap, it's just a bunch of useful CSS classes. The hotlink is a single minified CSS file.

I highly doubt the CSS included in the bootstrap is having a great massive impact on the performance of the new twitter interface.