With Emacs -Q open keyboard.c, scroll down so that line 89 reading

/* Variables for blockinput.h: */

is the first line of the window.  Now do

M-x outline-minor-mode
M-x hide-other

Here, Emacs consumes ~100% CPU and I have to get out of this via C-g.
Note: The problem does not occur when I call `hide-other' at bob.

The attached patch fixes the problem for me.