I'm guessing you didn't really mean that how it came out, but just for clarity -- It has everything to do with the hard drive actually. When a program needs more memory and it runs out of physical RAM, it uses the hard drive as virtual RAM (page file) which is much slower to access than physical ram and that is why the program "won't run as smooth."
Yeah I know, I meant if you have enough RAM you won't be using the hard drive as virtual RAM