It takes ages to download some emails. Over half a hour some days and if away for holidays etc much much longer
this can be caused by your email server, size of emails, your internet connection, limit of maximum connections to the server.
Can you tell me your internet connection speed and check if you are not downloading anything while synchronization?
Also if you use Outlook.com it might take a while. They has strict limit on synchronization (I think 50 emails per 15 minutes).