Hello orinoco,
I'm not sure if this is a graphics issue or some qt-kde issue.
xserver-xorg-video-intel defaults to SNA (Sandybridge New Acceleration), but your cpu is older.
The package contains an example xorg.conf:
mkdir -p /etc/X11/xorg.conf.d/
cp /usr/share/doc/xserver-xorg-video-intel/xorg.conf /etc/X11/xorg.conf.d/20-intel.conf
you can try to enable Universal Xcceleration Architecture (UXA), which is the predeccessor of SNA.
#Switch to intel with UXA
Section "Device"
Identifier "Intel"
Driver "intel"
Option "AccelMethod" "uxa"
EndSection
A more modern option uses version 3 of the Direct Rendering Infrastructure.
Xorg still defaults to version 2.1.
Option "DRI" "3"
greetings
musca