Network slows down dramatically shortly after connecting

I've decided to reimage my Dell XPS 9360 and started to experience serious networking problems right after the fresh Windows install. The previous OS installation became unstable but I had no network speed problems at all.

The device is equipped with Realtek USB GBE Controller, which means I need to use the USB adapter to connect the network cable. I can also use the network port built into my TB16 dock, but those docks are known to cause problems and they used to have a bug limiting network speed to 100 Mbps only.

OK, what happens - I install Windows, then latest version of drivers available on Dell's website, and finally run Window Update. When I first connect the cable I am able to get my full expected speed - 500 Mbps, but shortly after the next test shows approx 200 Mbps and few minutes later it's only 80-90 Mbps and this speed remains.

When I unplug the adapter or disable/enable network adapter, for a while I still have 500 Mbps and then the story repeats.

It looks there's some very recent update component that causes this - with my old install I had full speed, and I did a test by measuring the speed before running Windows Update - and the speed was around 500 Mbps constantly.

I've applied some common ethernet optimizations (e.g. turn off offloads, disable energy efficient enthernet etc.), but it wouldn't help - the issue is clearly linked to some Windows component.

Anyone can advise me on this? What else can I try?

Thanks, but the problem has been with me for a few days and I've already played with all power management settings that might have impacted the network speed. No improvement, though...