I'm disappointed this thread has steered into 'solutions' with VPNs, different torrent clients and random configuration which doesn't solve any of the issue that this thread was originally created for.
The problem is that in modem mode, and only in modem mode, loading up connections implements a limit on network traffic being able to go through the SH3 at 10mbps. This is easily replicated using torrents, but is not the cause. P2P by it's architectural nature creates lots of connections, be it TCP4 or UDP4, and thus has lead to this issue being noticed. I have tested this without specifically generating traffic from a P2P source, and the nature is still that *only* in modem mode, does traffic slow down until those connections are no longer established.
Some of the 'solutions' using VPNs may give you the behavior of this being fixed, but it's actually just down to the fact that OpenVPN, Wireguard, IPsec, PPTP etc will pipe your traffic down a limited number of streams, and hold your connection speed down to how fast those individual streams are.
I use my own networking kit, and aim to only use VM's equipment as a modem. Naturally that's the perfect documented use case for modem mode - apart from all of the issues posted on the earlier pages of this thread. I have temporarily created a workaround, that includes the exact same user scenarios (p2p, torrents, large single http/tcp streams) that work perfectly in 'router' mode (while still using my own kit, but double natting and placing my edge router in the DMZ). But fail using the exact same scenarios in modem mode, using the same kit. I correct myself here and I now don't believe it's a limitation of the SH3's hardware, as it obviously works perfectly in 'router mode' but a bias on the upstream network dependent on which mode the SH3 is in.
There is a definite association on VM's network of 'generic users' in router mode, and 'advanced' (potentially heavier users?) in modem mode. This is most obvious as the SH3 assigns different IPs, and different routes when in these modes. I'm tempted to think that it's a badly executed attempt of traffic management.
I'm also sad to hear that it's the exact same behavior on the SH4, further pointing it to be something upstream making this (purpose-built?) bias, or a bug that has never been fixed. I'm in a gig1 area and would've loved to upgrade, but I just can't justify paying for a service, which cannot provide me the same level of service down to the fact that I'm using the SH in an 'advanced' way by enabling modem mode.
I'm also interested that Virgin Business doesn't adopt the SH for their connections - and I've seen no evidence that this occurs on Business plans. There are obviously VM staff listening in on this thread to which there has been no official response, so rather than respond on why the network is so bodged. Can you answer on whether I can change my SH to a business-standard router? I'm happy to pay.
Yes, I could. But that's not the point. I'm using a residential service for residential purposes. The hardware right now does not work in 'modem' mode, but does in 'router' mode? I'm disabling all router functions while in that mode to replicate the 'modem mode' software function, with the compromise being that I allow the SH3 to do the initial NAT. I should not have to spend more on a business plan to get the service I am paying for which is supported by my residential plan.
Atissue's summary of the problem is fully accurate. At the moment I'm using the workaround of disabling UTP and only downloading a single torrent at a time to reduce max connections. I now get between 10-15 MB/s on average, although the connection will sometimes drop to around 5 MB/s for brief periods. This isn't ideal of course but until BT ultrafast comes to my area, VM remains the only viable option. I wish they'd acknowledge this issue and develop a fix because we simply aren't getting what we paid for at the moment.
I took a trial of airvpn early this year and hit the full 22mbps from my 200Mb line with newsgroups, if I switch to astrilvpn and use wireguard protocol I get 22mbps with torrents and newsgroups, without any bad packet errors or 10Mb throttle issue.
Maybe once openvpn 2.5 and the servers are finalized it may improve, otherwise wireguard is still the best option for myself.