bug#6810: 23.2; dired-omit-mode makes dired slow

[Leo]

When dired-omit-mode is on, operations on marked files become much
slower. To see this, following these steps:

1. Emacs -q

2. eval:
  (require 'dired-x)
  (add-hook 'dired-mode-hook 'dired-omit-mode)

3. C-x d and enter emacs/lisp (or any dir that has a few dozen files)

4. Mark all files by typing: * / t

5. Change mode to 444 by typing: M 444 RET


For every marked file, a message is displayed something like:
   Redisplaying...xyz.el
   Omitting...
   (Nothing to omit)

Can this inefficiency be rid of? Thank you.

Leo