Using Oulook over a VPN connection can cause problems if any of the following are true;
1) The internet connection is slow. In which case you can't do anything to prevent the freezes ups.
2) The user has a lot of email. You can archive old emails and emails with large attachments.
3) The user hasn't synced in a while or the user often receives new emails with large attachments. There's nothing you can do in this case.
4) The OST file is corrupted. Delete the OST file and let it be recreated when outlook is reopened.
Hope that helps,
tenaj