Rather than asking why the mac is so fast, the correct question is "why the hell is dhcpcd so slow?". There's a full second before it does anything.
replies(2):
Then the client will choose the best offer and will tell that DHCP server it took the offer.
I think after that the DHCP server is letting all other DHCP servers know the current status of available IP addresses. Maybe that will also take some time (with a lot of waiting, time-outs and stuff).
But then the question remains why should it take more than 2 seconds?