If you use it with every basic extension, like Kubuntu's default setup, then it's really bloated (Akonadi and MySQL is a dependency for KMail), but thankfully it's highly modular and enough fast for my taste.
I’ve not used Kubuntu in years but there was a time when it was consistently running 50% slower than every other KDE orientated distro (and some distros that were DE agnostic too).
I think Kubuntu did more harm than good for people’s perception of KDE.