Fix slow Start-up or Shutdown issue for Snow Leopard
Here is a quick tip that might help you if you are having issues with Snow Leopard slow start-up or shutdown.
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Slow start-up or shutdown can be caused by the kernal prelinking failing, because the root account is not assigned to the startup volume’s root directory.
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The fix is Simple
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Open Terminal and input the following commands | hit enter after each line.
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sudo chown root:admin /
(type your password and hit the return key)
sudo kextcache -system-prelinked-kernel
sudo kextcache -system-caches
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You should now see improved start-up and shutdown speeds, if not then your problem is caused by something else sorry.

It worked for me!
Thanks!
How did you discover it? Why does it happen?