macbook pro – Ethernet Thunderbolt 2 USB-C : What speed?

Over a lazy weekend, I find myself wanting to connect my fairly new MacBook Pro, with its USB-C connections, to wired ethernet coming off my cable box. Nominal cable speed is 200Mbps; I regularly measure speeds of up to 250Mbps.

However, unfortunately I do not have a direct Ethernet to USB-C. And — re: lazy weekend — I can’t be bothered to drive to a store to buy one (e.g. this Belkin). However, rummaging around my box of ancient, obsolete, and otherwise weird crap, I find I do have:

  1. An Apple A1433 Ethernet female to Thunderbolt 2 male connector; and
  2. An Apple A1790 Thunderbolt 3 female to USB-C male connector

Bolted together, those two give me Ethernet female to USB-C male.

Trouble is that, while that does work in terms of connectivity, and while each of the components should be well capable of handling the nominal 200Mpbs coming in from the street (and, as I say, I often get up to 250Mbps), it’s only giving me around 60Mbps.

So, any idea what part of the setup could be causing the speed degradation?