In my experience sluggishness comes from the crazy swapping that's going on all the time.

The N900 is set to the highest swappiness level by default, meaning it will favor swapping out to flash memory rather than using RAM that was just holding cache.

As root, do
It should have no ill side-effects and sets the swappiness level to the lowest. The N8x0 (Diablo) is using this level by default e.g.

After reducing the swappiness level, I experienced a much more responsive device, especially with several applications open running for a few days, and it looks like it has increased battery life, too.