I know what you're talking about. The culprit more times than not appears to be CB's tab behaviors, which load all of a profile's content upon the initial request. Caches might store up the queries that pull content, but it's still a big document.
The way to resolve this would at this point require quite a few changes in the core, maybe using Mootools (a Javascript Framework included in Joomla 1.5) to handle tabs and AJAX calls. But that would be very difficult to do if one of the primary goals of the project is backward compatibility.