Its probably the angry mob getting on all at once for the cake xp bonus.

The population has been steady and fixed at a lower level, so for a long while they had cut back on the servers to save money. Now that so many people have rolled in, the server quality they had can no longer handle everything.

They aren't prepared for this many people, and were still recovering from the blast of people that came with thieves guild. Its like launch all over again.

Just be patient and give them some time. Im certain they will have it fixed sooner than later.