I upgraded from 13.2 to 42.2 a while ago because I needed more current packages. I generally like the look and feel but it was SLOOOOOW. If I click on a menubar item, I would think I missed and click again because the window was so slow to appear, but no it would appear and then disappear again because I had clicked twice. Thats slow. I was seriously considering a different desktop or distro. I tried a great many things but today, on a hunch, I switched the compostor from Opengl 3.1 to XRender and restarted X. The difference is like night and day. The desktop is now fast enough to be comfortable to use. KDE is pretty heavy, but that weight is usually worth it.
I guess this is a notebook so you probably have Optimus graphics (hybrid Intel+NVIDIA). If so you need bumblebee. You may be running on one of the fall back drivers