A common problem in comparisons like this is that people compare what GObject gives them with C++ (and go ranting on how this or that is ugly in C++ ). That is is the wrong comparison, as Qt is not just a set of libraries, but a full-blown framework (even more so than GObject/GTK) - if you're using Qt, the valid question is if Qt addresses the problems C++ has/had, and often the answer is yes, like in the case or various monitor patters, reference counting, etc (in fact, sometimes I feel as if it's a C++ compatible C derivative of it's own, given all the syntactic sugar and patterns it introduces)
Qt is not particularly slow on N900 (I have no reason to think Hildon would be any faster). Even startup time is quite tolerable, less than 3 secs for a small Qt application.