It DOES make more sense that you cum quicker on your own. I mean, you are solely focused on the task of getting off. You want it done, and you're doing everything to get it done. When you're with a partner, you are (hopefully) splitting your concentration between your pleasure and making sure they are pleased as well, probably holding back to make the experience last longer, etc... There are a lot more factors in sex. With maturbation, only one.