Then I bought a new router - and Asus RT87U, and installed ddwrt, which is what my old TP-LINK router was running.
I sometimes work from home during office hours, and quite a lot in the evenings so we've setup an L2TP/IPSec for that, which has been working flawlessly up until recently...
When I installed my the new router and set everything up, and rebooted everything a few times I did a speedtest and woohoo - over 200Mbps.
But now my vpn perf is terrible - as in about 0.2 Mbps as measured by speedtest.net
I suspected the router so swapped it out for the old one - still the same. I then shared my EE android 4G connection to a laptop, and connected to the work vpn through that and the speed was usable at about 30Mbps. So the issue seems to be with either the SH3 (in modem mode) or Virgin.
Thanks for the tip. Unfortunately I'm not running the same firmware that article refers to and I'm not using a vpn service, I'm using the vpn provided by the Draytech Vigor router I setup at work. The problem seems to be somewhere between the ddwrt router and the virgin router when in modem mode. I've also swapped back to my old tplink router and that also now exhibits the same problem. The only thing that's changed is the modem...
Thanks for taking the time to post on the community.
My apologies that you are experiencing issues with your broadband connection.
I've managed to locate your account, the line and hub are showing as fine. The power levels are within range, there are no faults reported and the network segment is running stable.
How is the connection in general when you are not connected to the VPN?
I do find it change that you get this issue whilst in modem mode and router mode is fine. It does show that you are currently on the old firmware and hopefully when the new firmware is available this may help with the issue.
I'm at work now so can't check this exactly but in the end I changed the mtu to something less than 1500, by using ping's packet size and don't fragment options to compensate for the extra IPSEC data in the packets whilst the modem was in router mode, then switched it back to modem mode and it appears to be ok.