Comment # 7 on bug 871798 from Michal Srb

The slowness is caused by those factors:
* udl driver doesn't support hardware cursors -> X server has to switch to
software cursor and then every mouse movement triggers repaint.
* In this kind of setup, udl has output provider role in X server. In that case
it receives full screen update each time main GPU renders anything.
* udl driver doesn't keep a shadow framebuffer to find out which parts of it
actually changed -> it has to send the whole framebuffer to the device.
* udl driver doesn't support Display Link's proprietary compression (because
it's not well understood), so it uses only simple RLE.
* The device is slow.

Improving any of those would improve the rendering speed. Adding the shadow
framebuffer to udl would be probably the easiest one. Technically there is no
bug anywhere, udl has just poor performance in it's current implementation.

The slowness can be partially workarounded by using lower resolution and more
RLE friendly content (e.g. single color desktop background).