What misterc says.

The N900 is a mobile computer first and foremost. Phone call functions were an afterthought. It's bound to get clogged with temporary files, browsing history and message data.

Windows does the same, so we have CCleaner to clear all the left over junk that gets left behind after a day of surfing, gaming and whatever else we do. But because we don't have CCleaner for Maemo we have CleaN900 and Swappolube which go a long way to keeping things running well.