bug#22090: Isearch is sluggish and eventually refuses further service with "[Too many words]".

[Alan Mackenzie]

Hello, Emacs

With a recent emacs-25 (last update
eaa1fd6dbff8346eb38485de5ebf0fbfacf374d9 from Thursday 2015-12-03):

emacs -Q
C-c C-f src/xdisp.c
Move point to L30 (paragraph beginning "Updating the display is triggered
  by the Lisp interpreter ...")

C-s
C-w repeatedly, to yank words onto the search string.

After ~29 words have been yanked, the response becomes sluggish, pausing
for between 0.5s and 1s before highlighting the "for" at the end of L31.

Carrying on with C-w, some words are taking 2 or 3 seconds to be
registered by Isearch.  This is Bad.

After having yanked "you as part of" from L32,
(i) the " of" gets highlighted in the isearch-error face in the echo
  area;
(ii) the text "[Too many words]" is appended to the echo area;
(iii) the highlighting is removed from the match;
(iv) point is placed at the start of the match (i.e. BOL 30).

At this point, <BKSP> will still behave as expected, except it's action
too is very sluggish - to remove two words from the current search took
several seconds.

Observation: it may be that C-w done in the vicinity of two or several
spaces experiences extra delay.

-- 
Alan Mackenzie (Nuremberg, Germany).