Posting this because we had a weird thing with Turkey Day seasonal where splitting up to get more spawns and hit the end faster was arguably optimal. Splitting for more spawns does NOT WORK in the Christmas seasonal and can actually be extremely detrimental. The quest is designed for everyone to contribute towards killing any enemy that is currently spawned in the instance (outside of Umbra, obviously!), especially due to the Ultra Hard health increases.
The reason is because the Christmas seasonal can only have 1 set of mob(s) spawned at a time. If there are things spawned, you should be fighting them -- not running around trying to spawn more things, as it will not work, nor will it make things go any faster. And again, things in this difficulty have a TON more life than before.
Wanted to get this out since I feel like pugs are very likely to end up splitting up at first, and may not realize that it's a bad thing. If you see someone doing this, please don't be a dick to them either if you want to tell them that they shouldn't, just give them the reasons without being rude.
If you want to see some example runs of this quest ahead of time, feel free to look up PSO2 Christmas on Ice 2019 runs on youtube. The rewards, layout, and format seem to match up 1 to 1 with what they've told us we're getting.