So looks like it’s the former: Mastodon is simply overwhelmed right now because of new users. Hence cross posting is super slow.

To all users: PLEASE BE PATIENT. To fedi server owners: you’re not going to like what I’m about to say but it’s time to seriously spend money on servers and bandwidth. Mastodon is about to become real competitor to Twitter and we have to be ready for new users. #TwitterMigration