Note, KDE didn't work hard on optimizations lately, because they are focused on the deep redesign of the whole underlying structure. For example log in procedure in KDE is ridiculously slow. I have a high end machine (4 cores with hyperthreading, high end GPU and 16 GB of RAM), yet logging in feels very long and for a while the desktop is simply unresponsive. Obviously hardware is not utilized to the full potential to speed that process up. I expect them to start working on optimization as a next step though so I don't mind. KDE team is doing a great job and I'm gladly using KDE every day as my DE of choice.