One thing to remember is that overclocking isn't some magical fix. It can only do so much before the N900's other hardware becomes the limiting factor. There's also a bunch of system parameter optimizations which would effect your UI and visible speed/responsiveness more than the simple increase in clock speed, at least in cases where slowness wasn't due to processor speed in the first place, but was instead a RAM or Swap slowness problem, or whatever else.
Overclocking alone doesn't work what u need is to fine tune some kernel parameters. There are plenty of scripts, but u can try my collection if u want:
