dog-slow ediff in NTEmacs21.1

[David Abrahams]

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In GNU Emacs 21.1.1 (i386-msvc-nt5.0.2195)
 of 2001-10-22 on buffy
configured using `configure --with-msvc (12.00)'
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: ENU
  locale-coding-system: iso-latin-1
  default-enable-multibyte-characters: t

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

I just upgraded to NTEmacs21.1 from 20.7...  Moving from one diff to
the next can be unaccountably slow on some differences all of a
sudden. Turning off auto-refinement has some effect, but it's still
noticeably slower than in 20.7. In fact, this happens even if I've
visited all of the diffs in the file already. The delay appears to be
roughly proportional to the number of characters in each region.

For a reproducible example, compare the files at the following two
links. Moving to the 3rd diff produces a long delay.

http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/~checkout~/boost/boost/boost/
iterator_adaptors.hpp?rev=1.54&content-type=text/plain
http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/~checkout~/boost/boost/boost/
iterator_adaptors.hpp?rev=1.40&content-type=text/plain

This occurs even if I run emacs with -q, BTW. My ediff files appear to
be byte-compiled.

Recent input:
SPC SPC SPC SPC SPC q y C-x k <return> C-x o M-x M-p
<return> <return> M-p <return> <return> | @ h SPC SPC
SPC SPC SPC SPC SPC SPC SPC SPC SPC SPC SPC SPC SPC
SPC SPC SPC SPC SPC SPC SPC SPC SPC SPC SPC SPC SPC
SPC SPC SPC SPC SPC SPC SPC SPC SPC SPC SPC SPC SPC
SPC q y M-x M-p <return> M-p <return> M-p M-p M-n <return>
<return> y e s <return> | h SPC SPC SPC SPC SPC q y
M-x r e p o r t - e <tab> <return>

Recent messages:
Region 2 in buffer B is empty

Region 2 in buffer B is empty [2 times]
Refining difference region 3 ...
 [2 times]
Refining difference region 4 ...
 [2 times]
Refining difference region 5 ...
 [4 times]
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