I had also faced the same problem. Whenever I tried to disconnect from the VPN (in my case it was PPTP) server, KDE Network Manager takes 100% CPU. I used to kill KDE4 from "System Monitor". But even getting there takes a lot of time because the whole system behaves slow to any activity.
Found quite a few bugs reported for this issue. The one I mention here will give us more details about the issue : https://bugs.kde.org/show_bug.cgi?id=268038
To solve the issue, about a few days back I had to check on the "pre-release updates" from software souces-->origin under Kpackagekit. Several updates were installed and the issue got fixed.
From the above ticket it seems the updates are already there in Kubuntu updates and you probably don't need to install those pre-release updates. Just check whether there is any update available for you or not.
In case there is no available update from the regular channel, then you might want to try the above process as tried by me. But before proceeding please note that as per my knowledge, pre-release updates might bring in other issues. So please be sure before installing the pre-release updates.