MT hardware routers have old issue with low and unstable speed via VPN tunnel on high latency WAN link: viewtopic.php?f=2&t=146665
I also create ticket SUP-22475, but MT support refused to fix issue.
Do you want to know what reason they gave?
There is nothing to fix, it is the nature of TCP protocol.
Yes, they are honestly lying.
Testing shows that the issue occurs only in hardware MT routers, and it does not exist in the virtual CHR.
But they refuse to admit that there is a issue, and answer "There is nothing to fix, it is the nature of TCP protocol".