This isn’t a database issue, my guess is that it loads relatively fast from the database. The main issue is the amount of items and probably collections. The clients seem to have some issue what a large load of items. They all need to be decrypted to be able to be seeing shown.
And yes all clients need to decrypt the data. Everything is transferred as encrypted data, nothing is decrypt or encrypted regarding vault items at the server side, all is done at the clients.