it is the GPU, in all honestly it's slower because it is a nvidia and it's quite an old one at that. If you want to upgrade, from what I understand AMD cards are the best to go with. However if you want to stick with nvidia and are willing to upgrade at a reasonable price. I'm getting about 90Mh/s with a GTX 560ti from EVGA, not the best, but definitely better than what you're getting right now.