I'm seeing this from both sides, being that I'm the slowpoke in my group. You want a workout and you're not getting it with your current riding buddy. She wants a leisurely ride. Just remember that she probably feels intimidated by you, just as you feel frustrated.

How close are you with her? You don't have to be honest, but you could tell most of the truth. Like "I like riding with you, but I'm also training for (early-season event) and I'm trying to focus on that right now. Perhaps in spring?" and offer to do something socially with her. The event gives you some specific goal. If the communication thing doesn't improve, then you're going to have to find a way to cut ties a little.

Oh, and if you do ride with her again, take the beater bike.