I have tried smart queues on and off - no difference. The VPN is terminated on the client at my end - this is why I am finding this so confusing. Someone else has suggested it may be my ISP throtttling VPN traffic which actually would make more sense than the USG processing power to my mind. :(
{edit} mind you - that would not impact local network traffic so that cannot be it !
Jumbo frames not (and never has been) enabled. Yes, I think my next investigations is going to be the VPN itself. If I ever track this down I’ll post back just for closure :) Driving me nuts!