I've got the same problem DaZ is experiencing. Example? The screen will load the top part of this page, with the AnandTech Forums logo, and the top menu bar (y'know, the one with customize, profile, et al).
Then the IE Globe will stop and then it sort of just sits there. A few seconds later, the rest of the page suddenly reappears. Real annoying on a dial-up.
Another thing, when there's java on your immediate screen, things tend to slow down. Like the top logo, when that's showing, and I want to scroll, it takes an extra milli-second or two-hundred to scroll down. As soon as it disappears, scroll speed is normal. I don't have this problem at all with pages that don't use such obvious Java. I'll have to try out BetaBites and see if having their menu slows things down.
Most of the improvements were at the developer's level. New support for tags, and that sort of thing. The part I like about IE5.5 is the print preview. But I'm not interested in sacrificing speed for a feature that I may or not may use. I'm going to downgrade to IE5.01.
(Oh... and IE3 was the best. )