Open terminal, type top and hit return.
On the last line of the header (before the columns), look at the part that says:
23432(0) pageins, 2343(0) pageouts
Your numbers will not be the same. Now, leave top running and do stuff on the computer.
If the numbers in the () are always changing, then that means that you are constantly swapping things from the disk into and out of active RAM. This causes slowdown.
Remember, it is the numbers IN the (). It is normal for the numbers to the left to be large as they represent the total numbers of pageins/pageouts since startup.
Hope this helps...256 is almost certainly the edge of what is really needed.