When synchronizing an IMAP account for the first time, it can take a while to upload & download the existing email history to our mail servers. The time it takes depends largely on the upload speed of a users network & the number of emails it needs to sync.
Another potential solution would be to configure your IMAP account and leave it online overnight, so the email client can push & pull the data to and from our servers while you are away for the evening. When you arrive back at the office in the morning, your files should be fully synchronized.
Some email clients also have a "headers only" download option, so it won't download the entire message until you click into it. This can dramatically speed things up.
Slowness with syncing an IMAP account
