What is the fastest VPN?

The scenario: I allow mobile laptop users the ability to access files on the server from their homes (and other off site locations). We have an older Watchguard XTM firewall and they use the Watchguard SSL VPN client. The firewall has reached end of life and is up for replacement. The biggest complaint I've had is that the VPN is too slow. (It is only used for internal network traffic). THe office internet connection is business class cable (Cox) 25 down /4 up.

So my questions are:

1) What is the fastest way to allow users to access files on the server from remote locations? I'm open to non-VPN suggestions.

2) where is the bottleneck? Assuming their home internet is faster than the work connection - is it our internet service, the XTM throughput (40Mbps), or just VPN in general.

Thanks!