when my wife first started riding, she was slow, thought it was just because, (in my best sexist attitude) a women, well once I got her fitted to a proper bike that all changed. Now the issue is keeping up with her.
That said, if your GF is really interested in riding, make sure she has the right tools. My wife even works with a coach during the winter to keep her ability sharp.
It great to have your lady along on a group ride and see them all look perplexed at who this women is that just won't go away.