I've got a Mikrotik Routerboard hooked up to a (Snap) VDSL connection via a Vigor VDSL router in bridge mode. Overnight my observed speed dropped substantially, and after hunting around for a while I've noticed that the Mikrotik is reporting fairly heavy transmit bandwidth usage (3-5Mbps). The link speed from the Vigor is sill about 26 down/7 up, which I think is quite reasonable and is consistent with what I normally get.
I've gradually unplugged everything on the network side of the Mikrotik without seeing the Tx usage drop. This suggests to me that the Mikrotik is doing something stupid with the connection. Or perhaps someone's compromised the router?