I haven't had a single problem with my N900's speed since I bought it back in September 2010.
Then, a few days ago, I shut it down for the night (to preserve battery as I wasn't at home, which I had done quite a few times the past week) and booted it back up in the morning.
And since then it's SLOW. Transitions, touches, closing programs, ...
And I haven't changed *anything*.
Top says the load is about 0.30. I haven't looked at it before, so I don't know if that's a lot or not.
Occasionally it acts sluggish, but it's only because an application (like email) suddenly thinks, "Oh look, an internet connection! Let's get that email!" and everything slows down in, say Angry Birds.
One thing you could try is restore from a recent backup, or turn off and remove battery for a few minutes, or pour Red Bull all over it. I tried the last one and it turned into an N9....ha ha