It’s the amount of users and servers in the room. When you join it on a new server it needs to process and verify the state of the room and agree it with all the servers. As the state is massive in this room it can take a very long time to join it.
If you want to learn more read this: https://matrix.org/docs/older/stateres-v2/
I’m pretty sure 2FA can work offline as well. You definitely don’t need an internet connection to perform 2FA with OTP codes.