[OS X Emacs] Slow vertical split windows viewing same buffer in Aquamacs 1.6

[Drew Hess]

I'm seeing a performance regression in Aquamacs 1.6. Open a file
with 50+ lines in it. (I've tried text, elisp, Lisp and Scheme files
and all exhibit the same problem.) Make a vertical split window with
C-x 3. Make sure the 2nd window is displaying the same buffer as the
first. Now move around the file with C-p, C-n, C-f, etc.; try holding
one of the keys down until keyboard auto-repeat kicks in, for
example. On my machine, performance is noticeably sluggish when I do
this. Each additional vertical window displaying the same buffer
impacts the performance more negatively, When displaying a different
buffer in each window, however, the performance seems to be fine.

When I create horizontal split windows and display the same buffer in
each, I don't notice any performance hit, unless there's also at least
one vertical split displaying that same buffer.

I'm pretty certain this didn't happen in 1.5.

thanks!
d