It may be than some people warm up very slowly compared to others. Something like that seems to be true of myself. Even back in the days I did considerable hiking and backpacking. I'd be wearing a parka for the first half hour while my hiking partners taking off extra layers within 10 minutes. On the bike it is similar. I'm asthmatic and get short of breadth very easily in the beginning. As the ride progresses I begin to feel better and can then pick up the pace. I've noticed that pros frequently use a trainer to warm up before competition. Perhaps you just need to spend more time warming up.