I have this problem too. From whatever research I've been able to do, I get the impression that it's physiological -- some people just take longer to warm up than others. I've tried to find tips on how to train yourself to warm up faster but I haven't been able to find anything. If anyone has any suggestions, send them my way!

I don't race, but if there's a group ride where I know that the warm-up period of the ride won't be long enough for me, I try to get there early to warm up on my own before heading out with the group. That seems to help when I'm able to do it.