External Display throttles Internet speed

Internet speeds collapse, to the point that it's too slow to run a speed test, when I connect my Macbook Pro 2017 (Ventura 13.2.1) to a new Eizo external monitor (CS2731). The connection is via USB-C. Different cables have been tried, both directly into the laptop and via an OWC Thunderbolt 3 dock. All combinations produce the same result.

Any thoughts on how to rectify this would be appreciated.

Thanks, C