Usually this issue is caused by slow Internet connection and / or huge emails.

To resolve this:

1. Try downloading your emails using IMAP, this will help in dealing with emails with large attached files.
This is so because the attachment won’t be downloaded automatically, which is the main reason for slow mail download when using POP3.

You can also log into webmail: yourdomain.com/webmail or webmail.yourdomain.com to see what emails are trying to download.
Delete the biggest emails with attachments bigger than 5mb after downloading via webmail.

2. Check the speed of your local connection and ask your ISP for additional assistance.
Make sure nothing else is being downloaded.