It does appear that there is a problem somewhere. In order to be able to do anything about it we first need to locate the problem. This means that we need a very detailed description of the network setup, even up to details about the physical devices used for the connectivity (exact model of the router or any other devices used for the connectivity).

I can understand the frustration, but we really do need good reports or we can't do much about it.

Unfortunately locating and fixing the bug can take some time. Meanwhile, if you happen to have some spare switch around it might be a good idea to check if connecting the ethernet thru it changes anything. My current guess is that it might be a problem in the autonegotiation. If you find any way to fix the transfer speed problem (by using a switch or by any other means) then please do tell us. This information would be very helpful.