Backing up to do clean install

You will definitely notice a speed bump.

I did a clean upgrade on a 1.5 yr old SL machine and it was like having a new machine. For 30€ that is one hell of an upgrade.

You can backup Documents, or your entire User folder (would include Music, Photo libraries).

For settings you can backup the Library folders (one in your user folder, another one in the root of the hard drive), although I chose to start anew with everything. You could also goto Library and look through Preferences, Application Support and see per-app settings you want to save.

Use the opportunity to manually install Applications. From well over a hundred in SL I went to under 20 in Lion. I figured out that I could easily download the app the one time every year I use it.

Other things are - Safari bookmarks, passwords, Keychain, Network settings, iCal calendars, Address Book.

I doubt that doing a "clean" install will make your machine any faster than doing a regular install.

If you have tons of stuff on your Mac that you shouldn't have, and have processes running that you shouldn't have, it may benefit you…but so would being more conscientious about running applications, extensions and other things that might slow you down.

If you intend to erase your hard drive, install Lion, and then manual move stuff from a backup please make sure you know what to move where. Easier to just migrate from a time machine backup. Or, better, clean out the crap BEFORE you back up and make sure it's running OK, then back that system up and upgrade.

Rob