I just updated to 4.9 (it hit the Gentoo Linux repo yesterday.) KWin is a bit of a disaster :-| Too much lag. Moving a window around means the mouse cursor moves first, and only later does the window catch up to where you moved the mouse. Disabling VSync doesn't help (both in System Settings as well as in NVidia Settings.) It was perfectly fast and snappy in 4.8. My theory of KDE accumulating more and more bugs turns out to be true, at here here.