C++ is definitely a drawback for a long time. Not because it is 'bloated', but because of how ABI changes were handled. It is all history now.

Nowadays the major problem with KDE is that by the time it is stable a new QT major version gets released and along with that it essentially gets a major rewrite, which takes years to stabilize and once it does a new QT version is released, etc etc etc.

It depends. KDE3 under Debian was much faster than Fedora Core with Gnome 2.