[A complimentary Cc of this posting was sent to
Randy W. Sims
<RandyS@thepierianspring.org>], who wrote in article
<435556D7.7030303@thepierianspring.org>:
One wierd problem I find is if I load two copies of a certain file it
becomes develishly slow to do anything, even scrolling. This doesn't
seem to happen on all files; not sure if it's size or content related yet.
It looks like it is indeed a bug in font-lock.el. At least I put a
workaround for this bug in v5.7, and the slowdown (apparently)
disappears; see
ilyaz.org/software/emacs/
The workaround itself may slow things down, but I hope its effect will
not be noticable.