You can see by the page outs that you have exhausted the memory, so you are writing to disk.
I had 4gb on my last mac and was always getting slow down and plenty of page outs. Just bought a new one with 16gb and its lightning quick, always around 6-7gb free and 0 page outs.
If you mac can take more then I would always recommend it, just don't by ram from apple. Get it from crucial.