I have 9Mb text file with 90000+ lines in it. When opening this file in
emacs-24.1.1 and scrolling to line 5000 and onwards the scrolling
becomes very slow and there is a long delay after I press PageDown and
emacs updates the view.

I heard that font lock may cause this so I disabled font lock in that
buffer. Disabling font lock doesn't resolve the issue or speed things up
a bit.

The contents of the file look like this:

2012-10-09 08:35:28.217216000 167.150000  44035
2012-10-09 08:35:28.217216000 167.200000   2859
                                                 2012-10-09
08:35:28.217216000 167.250000  54888
2012-10-09 08:35:28.233155000 167.150000  66035
                                                 2012-10-09
08:35:28.233155000 167.250000  44404

(long line wrapped in the above).

I.e. there is a timestamp on each line of it in the form of YYYY-MM-DD
HH:MM:SS.nnnnnnnnn.

I almost accidentally replaced all 2012-10-09 with YYYY-MM-DD in the
file and that fixed the issue, the scrolling became swift again.

Do you guys have any clues what may be causing this scrolling slowness
and why replacing ISO dates with YYYY-MM-DD makes scrolling so much
faster? (once again I have font lock disabled in that buffer).

-- Maxim