low MH/s on guiminer(solo mining)

[Amph]

hi, i can't get why my mhash per sec is only 2.7 on guiminer while solo mining(i have a 670, so it should be around 100mh/s)
it's supposed to be that low, if you solo mining?

p.s. i know that isn't profitable, but i want try just for fun, to learn alla the things about bitcoin