David bought a used Mac Mini and he is trying to set up his Apple Mailbox. It's not moving the messages over fast enough, though. Leo says that David is trying to store hundreds of thousands of messages and that just takes time to download them. Why do that when he can keep them in the cloud? Also, the new Apple Mail has to reindex the mail and that takes time. This is why Leo is against this proprietary mail format. He prefers open source options like Thunderbird or with GMail.
Leo says it may be time to force a reindex by holding down the Option Key when launching Apple Mail, which will rebuild the index. It could also be in the menus. He could try deleting the index files themselves, but he should be careful not to delete his emails. This article from ComputerWorld explains how.