bug#41200: Displaying a tooltip with x-show-tip gets very slow as more faces are defined

[Stefan Monnier]

> Interesting.  I also wonder whether we could fast-track the case where the
> face spec is a vector full of 'undefined, since that seems to be the
> common case.

I was thinking also of the case where no face specs have changed and the
frame is "normal" (same frame properties as others, basically) so we
could re-use the faces from another frame.

[ After all, my Emacs sessions typically have dozens of frames, and only
  one of them (a minibuffer-only frame) has faces setup differently
  because it's configured to use a slightly larger font.  ]


        Stefan