Several popular graphics cards out there - especiall NVIDIA ones - exhibit performance problems with KDE4. These performance hits mostly concern Plasma - the KDE4 desktop - and KWin - the KDE window-manager. Generally speaking this isn't due to KDE4 being unoptimized but a problem with the cards' graphics drivers. This page encompasses certain problems that might arise and how to get rid of them.
This section summarizes several symptoms you might encounter and tries to explain possible causes.
$ XLIB_SKIP_ARGB_VISUALS=1 konsoleon the command-line and try to resize the new konsole window (setting the environment variable XLIB_SKIP_ARGB_VISUALS turns off ARGB visuals). If it resizes without problems, then this is certainly one of the problems you're facing.
$ nvidia-settings -a InitialPixmapPlacement=2This works for some (but unfortunately not all) NVIDIA cards.