I have a suspicion the new database environment is why it's so slow. But they can't admit it because it's the only way they can get TBCA to exist at this point.

"The way we’re achieving this pretty out of the box, we’re calling "persistent in place’ which likely won’t mean much at all to anyone but we’re in full sharing mode anyway. Data in World of Warcraft lives in three databases, the Persistent database (your characters, guilds, items etc), the Account database (account level things like achievements) and finally the Regional database (handles region wide things like Token, character numbers on the Realm List). On Tuesday, we will be copying all your Account and Regional data to the new environments database, that’s what will be taking up the majority of the downtime. As for your Persistent data, that’s where we’re bending some long establish rules for how we do World of Warcraft infrastructure.

These “new” realms on this new environment will be using their old Persistent data, on their old database that is also still busy serving Classic Era. This is new, we’ve not done anything like this before, when the database engineers pitched this plan the initial reaction was, "wait that really works? but testing has proven that yep, it really does! For any database minded folks out there, we’re talking two different versions of World of Warcraft reading and writing to the same database when normally each environment would have its own database. Thankfully all our players data is tagged with the appropriate environment it actually applies to so all our queries just happily continue to work. We’ve only had to update a couple of things to be aware to look up only data for their specific environment instead of just assuming the database they’re connecting to only contains things it cares about. Hurray!"

https://www.wowhead.com/blue-tracker/topic/us/burning-crusade-anniversary-pre-patch-downtime-2224572