Hi Richard,
What are the size of the files that you are trying to access across the VPN?
What is the other Internet activity consuming bandwidth of your connection?
If there is allot of Internet surfing, Video and streaming radio, you could be loosing the bandwidth that you need for the VPN.
Does your VPN equipment have any traffic shaping and priority handling?
You may benefit from an outside consultant to see if you are set up optimally.
On my VPN I used the bandwidth optimization feature which improved my performance greatly.
Dave