This is a strange issue. Do you have it set to play on fullscreen or windowed?
Defragging may help, but I'd honestly want to see your RAM/Process usage before I jump to conclusions... It could be that all his RAM's being OM NOM NOM'D by something, or his processor's being overworked on something.
Also, for those telling him to get a new computer, he's got a decent enough one. His video card and processor is better than my desktop's, and I run Blockland on there perfectly.