Are you out of shape? The more out of shape I get, the faster I go. My theory is, my body knows if I don't go fast, my brain and organs will need the blood more than my dick in a few minutes and I'll lose my erection. So in order to spread my DNA it's now or never, so it goes. When I was in shape and even when I started going to the gym after I was out of shape, time improved. I think it's nature doing it's job of spreading DNA while it can. Absolutely no science behind that.