this seems to be the same issue I have with vdsl. the ppp0 mtu rate seems to be lower than rated. if you do an ip link show at the shell you’ll see that mtu for ppp0 should be set to 1492. BUT if you go to http://www.speedguide.negt and let check from the outside you’ll get 1452 which seems to be is too low to handle unfragmented packets fast enough.
I have nothing found on this issue nowhere. And the programmers don’t say a word to this.
Hopefulle they get this issue managed