Hi Emil,,
The most popular problem with VPN connections are MTU issues. Try a ping -l 1500 internal-ip. If that doesn’t work, you have an MTU issue.
Try to decrease MTU size to 1300 and see if that makes any difference.
The next step is to test the raw speed – copying a file with FTP to and from a server. Is the speed okay? If the speed is according to the lines used, your problem probably isn’t with the VPN connection.
All the best.