Actually they build things really really fast... And they have some ready pieces outside the park... i believ that part of the purpose of Hogsmede is to counter disney's christmass parties, so it can be opened by next christmass.
Although i personally will prefer an all-together opening, it will be better, maybe Universal will wait with hogsmede until they can open the entire land together... this will make a great impact, better than opening in two stages- this will just dissapoint fans.