Maintenance for the week of December 8:
• [COMPLETE] ESO Store and Account System for maintenance – December 10, 9:00AM EST (14:00 UTC) - 12:00PM EST (17:00 UTC)
The initial log-in after a patch is always a lengthy one. It has been like this, with a heads-up from Gina prior to it happening, since the Greymoor update.
The game has to re-populate the database with all of your character data. So, side-note, the more characters you have, the longer it will take.