How to shorten startup time after Lion to Mountain Lion upgrade?
Once you've paid once for an OS upgrade, you own the right to download the OS again at any time. Reinstalling will not require another purchase. If you wish to try your luck with a fresh in a fresh install, consider watching TLDToday's video series:

How to Create a Bootable Mountain Lion 10.8 USB Install Thumb Drive

How to Clean Install OS 10.8 Mountain Lion