Upthread, fransick had reported some performance issues (map lag) but they were resolved by a clean install. I'm not sure if that's an option for you, but if so, you might give that a shot. I suspect there could be some sort of conflict between qt3 and qt4, or some other "cruft" that's causing an issue.
Are you running Raspbian, or some other distro? What version? Was QT installed via the package manager, or a different way? What version?
Upthread I gave instructions on getting profiling data. That may or may not help, but since I still haven't been reproduce any kind of performance issues, it might at least give an idea of the problem. I'll think about it some more and see if I can come up with any more ideas, but that's all I've got for now.