I have the Vigor Smart VPN client 4.3.3.3 connecting to a Vigor 3900 firewall, which is on a 100MB leased line connection.
If I connect the VPN from my office pc (windows 10 on an 80/20 FTTC connection) and browse file shares, the connection is slow and opening a 10MB file takes 4-5 minutes if it opens at all.

My colleague also in my office on a similar Windows 10 pc, has no speed issues at all. His computer browses the same file shares and opens photos from this share within seconds.
Any thoughts very much appreciated.
Many thanks

Is one using SSL and the other something else (e.g. L2TP)?

I've found SSL to the Vigor to be very slow compared with L2TP/IPSec - though, admittedly, this observation is based on Android phone usage.

Both are using the Smart VPN Client and both on 4.3.3.3 and both using SSL yes.
Really odd behavior.

This has been narrowed down to the SSL connection.
When using the Draytel VPN client but with a PPTP connection, the speed is as expected.
SSL VPN brings the connection speed to a halt!

FYI - https://www.draytek.com/en/faq/faq-vpn/vpn.others/security-and-performance-comparison-of-vpn-types-which-vigor-router-supports/