Only 100mbit Ethernet speed with an OWC Thunderbolt Dock

good, the dock features Gigabit ethernet.

Now about your cables...

to work at Gigabit speeds, a modulation technique is superimposed on the baseband data. This requires more spectrum, which requires ALL EIGHT conductors must be present, and wired in one of the standard configurations. Four conductor cables work great at up to 100 M Bits/sec speeds, then will go no faster.

[ I discovered this "feature" experimentally after a long-into-the-night debugging session. I was so annoyed, I went through all the spare cables on hand, and cut the ends off all the 4-wire Ethernet cables I owned.]

Your other devices, e.g., your Router must also support Gigabit Ethernet transmission speeds.

To get appropriately data throughput, you must invoke Jumbo frames on the Mac. Some devices have a little trouble with 9000 byte frames. I find for my devices, 8163 works reasonably well.

And to confirm: you’ve tried the same Ethernet cable in a different configuration, and had gigabit Ethernet working there?

This as older Ethernet cables and some broken Ethernet cables are limited to 100 megabit Ethernet.

what model OWC dock, or a link to its specifications, please.

You can pre-configure it to REQUIRE Gigabit speeds instead of allowing it to auto-detect the speed.

The risk in that approach is that it may not connect at all if it cannot attain Gigabit speeds.

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Your newly ordered cable should work fine, but is far more than necessary for this purpose.

If you view the internal wires through the clear connector at the cable-end, it should be easy to tell 4-wire cables from 8-wire cables.