It started to really improve after I think the 3.5 update, before that it was indeed pretty bad regarding performance. The more recent updates also seems to have finally fixed a lot of the more common issues that I was running into. But honestly, the grass is always greener on the other side. Tried Gnome recently again after many many years (pre Unity) and everything was just crashing or lacking very basic functions that I was using in my daily workflow with KDE. The only thing I liked were the smooth animations, but looking polished is not the only thing I need from a DE, especially when you rip out everything else to achieve it.