I have found a way to improve the performance massively: I have made a custom setting for the radeon driver. This is my
Section "Device" Identifier "Radeon" Driver "radeon" Option "SWcursor" "off" #software cursor might be necessary on some rare occasions, hence set off by default Option "EnablePageFlip" "on" #supported on all R/RV/RS4xx and older hardware and set off by default Option "AccelMethod" "EXA" #valid options are XAA and EXA. EXA is the newest acceleration method and its the default. Option "RenderAccel" "on" #enabled by default on all radeon hardware Option "ColorTiling" "off" #enabled by default on RV300 and later radeon cards. Option "EXAVSync" "off" #default is off, otherwise on Option "EXAPixmaps" "on" #when on icreases 2D performance, but may also cause artifacts on some old cards Option "AccelDFS" "on" #default is off, read the radeon manpage for more information EndSection
The key option that makes the difference is the EXAPixmaps, which increases the performance a lot.