$ emacs -Q /usr/share/dict/words

C-SPC

M->

^X^X  .....

(beep) Timed out waiting for property-notify event

 

You can recover by hitting ^G several times.

 

Problem appears when the transient-mark encloses a large region.  Does not occur in a terminal window (i.e., emacs -Q -nw).  Appears to be an x-selection issue.