under not exactly determined circumstances (nothing fancy - a non-themed kdm greeter is sufficient), the X server constantly sucks 100% cpu time. as a result, everything is abysmally slow (and events of all kinds get eaten, but i blame that on the stupid event queue which generally doesn't seem to be able to cope with high internal latencies).
Created attachment 26282 [details]
This may have been fixed recently. Could you try xserver master?
Please reopen if the xserver master still has this issue.