Dunno, personally, I subscribe to ShadowJK's theory of swap partition acting slow once it rolls over after a few days. This, and the swappiness=100 killing all the heavy apps like Fennec (once I set it to 30, Fennec, Video Player, and other sluggish apps become perfectly usable).

PS: Looked at the iostat results. Nothing special there, except maybe for swap writes grossly exceeding swap reads.