No uploading is going on other than the overhead of downloading which appears to be not high for the abilities of the connection: Only about 30-40KByte/s when the router locks at 1012Kb/s and other direct uploads or uploading overheads can reach more than 100KByte/sec so I don't think it's a congestion at uploading that is doing it.

Is there something I miss? Because I assume 13611Kbit/s should be ~1701Kbyte/sec.

Is it an overheard at the ADSL level I don't understand? Could it be the ISP doing it?

If it's active throttling it can't be on single connections since 2 high speed connections still go up to ~1500KByte/sec.

It's not an example on torrents or other complex situations.

The tests were on Ethernet, but I doubt the results would be different on wireless.

I wonder if the settings of those connections at my end could be doing it, e.g. MTU settings, though I haven't touched the defaults of a common Realtek NIC.