As above universities generally close over the Christmas period (the one I work at is closed from the end of this week until the new year), and of course staff may be taking their annual leave earlier if permitted so it might be slower in the run up to that anyway. As noted it's not really an answer anyone can definitively give you as the administration of different university staff contracts etc is likely to vary quite widely. Some may allow staff to take annual leave in term, some not - if the latter, more staff might be taking time off after the end of term but before the start of Christmas closures. There might be sickness or unexpected leave which means staff are off anyway....etc, etc.
Universities are contractually obliged to consider all applications received before the January 15th deadline equally, so there isn't going to be much difference between submitting it now and submitting it in the new year as long as it's before the deadline. Getting an offer earlier doesn't make any difference in the long run, and provided all your offers are received by the deadline UCAS encourages unis to make offers by, you will have the same amount of time to consider and respond to all offers. Thus, it doesn't make a difference what the answer is really anyway, as it won't materially affect your prospects as long as you submit it before the deadline.