Thats normal most of it is cached - mine uses 300 mb
As to being slow after a while:
I have the same problem with konqueror as a filemanager.After a while of after moving around a couple of gig's that thing just gets buttslow or dies completly - depending on the version of konq and the distro.
I tried to find out for some time what could be resposible for that but nobody seems to have the slightest idea.