qwt 5.2.1 has been released 2009 - and the only reason why it is popular nowadays is because of legacy code and its python bindings. Doing a backport of running code because of not being able to install a current Qwt version , hmmm ...
Beside that I don't see a general reason why a replot of the same plot should take 5 seconds with Qwt 5.x, while it runs fast with Qwt 6.x. What the specific reason iin your case is, is hard to say without having the application running.

IMHO it is a valid consideration if you want to go with Qt5 or Qt4, when starting a new project - but when going with Qt4 it doesn't make much sense with a version < Qt 4.8.

Uwe