HOW laggy? is it tolerable?
It can't even run a damn pixel game (desktop destroyer) Besides, it's nowhere near HD in quality. Just get the Fraps trial, record a bunch of 30 second clips, then put it together in windows movie maker.
EDIT:
I believe it is. Also, every screen recorder out there uses a considerable amount of memory.
Computer specs?
Looks like it just depends on your computer hardware... and what you consider tolerable.