If I had to guess I'd say that the poor performance is largely due to the low-end nature of the GPU combined with the fact that the driver probably has very limited capabilities. Take a look at the commit log for that driver:
http://cgit.freedesktop.org/xorg/dri...conmotion/log/
Almost everything from the past year are simple fixes to keep the driver compiling for newer versions of Xorg or simple code cleanups. This suggests that there are very few developers working on this driver.
I doubt there's much you can do to improve performance.
Adam
