As I have already flagged [1], current trunk shows a regression on
GNU/Linux Kubuntu 8.04.1 (and at least on Cygwin):

Moving with UP/DOWN arrow keys takes about 1 second to change line. This
happens with files greater of a certain size. I have seen this with a
file of 11MB but also with a file of 4MB or 5MB.

The attached file, 'test_file.txt.bz2' (342 bytes!), when decompressed
has a size of about 5MB and should show the 'phenomenon'.

Cheers,
    Angelo.

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[1] http://lists.gnu.org/archive/html/emacs-devel/2008-08/msg01384.html