When signing in on a new device you'll have to sync all of your previous messages that you've sent or received. We do this all in the background so there are no additional steps for you as a user to sync these messages but you may see some messages loading slower or some slow loading user profiles which is normal. You can see the total messages that you are syncing by clicking on the team switcher, at the bottom of the team switcher you can see the total messages being synced.