Display slowness that is painful

[Richard Stallman]

[I sent this message twice but did not get a response.
Would someone please look at this problem?]

I just displayed a large binary buffer which had many chars in the range
0200 to 0377.  And the region was large, too.  This led to a very long loop
in which I saw the following on the stack

    try_scrolling, Fwindow_end,
    start_display...get_next_display_element...handle_stop...handle_face_prop,
    face_at_buffer_position, merge_named_face (on Qregion)

... means I omitted some intervening function calls.
I did not have a way to actually save the backtrace, so I typed
in enough that I think people can see what the issue is.

It seems that this case badly needs to be optimized.
Can someone please work on it, then ack?

I can make the test case available if necessary.
It is a 1.3 meg GIF file.