No subject

  With xcb every write (in _XFlush) is followed by a read
  (_XEventsQueued) which is the cause of the very poor performance. The
  read usually returns EAGAIN because there is nothing to be read, but
  it is a severe performace hit anyhow.  And why _XEvenetsQueued/read is
  being called when select/poll has previously returned indicating only
  a single file descriptor was ready is anybody's guess.

Can you provide some insights into what causes this issue?

These seem to all (maybe?) be reports of the same issue:
  https://bugs.launchpad.net/ubuntu/+source/xulrunner-1.9/+bug/305531
  https://bugs.launchpad.net/fedora/+source/libx11/+bug/277069
  https://bugzilla.mozilla.org/show_bug.cgi?id=475713
  https://bugzilla.redhat.com/show_bug.cgi?id=442158
  https://bugs.freedesktop.org/show_bug.cgi?id=17868

Thanks,
Bryce