This is rather weird.
I'd had Ubuntu on both my Emachines (ER1401/2) it ran OK but a bit laggy in places even with the correct Nvidia drivers installed.
Video playback using VLC teared like a b***h so I had to mess with the Xorg config to disable composite just to get smooth playback, I had good playback but Unity looked awful.
Installed Kubuntu a few days ago, both are running extremely well, no random lagging & video playback is smooth as butter both before and after I'd installed the Nvidia driver. Work that one out

.
I'd tried Kubuntu before & I never really liked it but now they've made so many improvements I actually quite like it, The Muon Package Manager is excellent too, much better than the god awfully slow Software Center in Ubuntu.
For me Kubuntu is starting to over shadow it's big brother, and I never honestly thought I'd say that!
Performance wise Kubuntu is definitely much faster for me than Ubuntu on the above machines.
Yay choice!