The N900 is supposed to be a very fast device, even without overclocking.

I recently received a new unit as my old one had a few warranty-covered problems they were unable to fix. For my luck it came pre-installed with PR1.3, so I'm probably one of the few that have a vanilla PR1.3 image installed. The device is lightning fast, browser works faster than ever, apps launch in about 2/3 the time they did on my previous unit. The previous unit was OC'd for the entire time it's been possible with easy instructions.

So here's my point: don't overclock unless you're interested in the PlayStation emulators and want some extra kick to play those games. Reinstall the image (flash your device) and you'll be fine. If you want to spice up your device, find the discussion on "Custom transitions". This will make your device seem even faster.

The key to a fast N900 is simply the "purest" install you can muster.