https://bugs.freedesktop.org/show_bug.cgi?id=74190
--- Comment #1 from slicksam@gmx.com --- This may be the same issue I reported in https://bugs.freedesktop.org/show_bug.cgi?id=74140
If you check CPU usage, you should see a thread stuck at 100% - this is apparently it trying to compile the opencl program. I have not tried waiting a long time to see if it does eventually fire up.