I have two users on an i7 Intel based workstation with 16 GB RAM, SSD system disk, and an AMD r600 GPU ... one user is using the SUSE KDE theme and the other user the original KDE theme ... neverthelesss ... beside the annoying slow login (what does KDE/Plasma do during login????) there is a  behaviour of the control bar at switching the working area.

By clicking on a work area in the pager of the control bar it takes several seconds to see the tasks in the taskbar belonging to the work area ... btw. clicking into tho root screen triggers the refresh whereas not clicking let one wait several seconds. Beside this if there are package updates via packagekit available and shown in the control bar the time of refreshing the control bar increases.  Looks like a linked list of graphical objects are created but only shown by luck.

Meanwhile I've disabled all the useless graphical gimmicks for both users but this only speeds up the windows like firefox, konsole terminals, ... and their behaviour at popup.

IMHO there is a bug in the refresh in the control bar or part of the control bar of the plasma workspace. Means if a user switches the work space then the control bar should follow mmediately ... as e.g. with fvwm and its pager.

Beside this glxinfo -B, vdpauinfo, and vainfo as well as the log files of the Xorg server shows that the AMD GPU is used and does its work.