I just checked out Plasma 5 in Kubuntu 15.04 and experienced changes with xrandr are really slow (compared with Plasma 4 in Kubuntu 14.04 LTS):

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xrandr --output LVDS1 --mode 1024x768 --output VGA1 --same-as LVDS1 --mode 1024x768


takes about 10 sec where Plasma 4 took 2 sec until all decoration and windows are resized and placed.

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xrandr -o inverted


works, but leaves the screen black - until I switch to a tty and then back to ':0'.

Are these effects characteristics/shortcommings/bugs of Plasma 5?

This is a bug in Qt, unfortunately. It may be fixed in the next release (Qt 5.5) when it's out.

Actually it's a (IMHO security issue) in the driver/X11 stack. There's a call to query hardware which blocks the X-Server. When xrandr performs changes all Qt applications do that call and each one blocks X11 from functioning.