9360 network performance after Windows reinstall

I've just reimaged my XPS 9360 with the fresh Win 10 x64 Pro copy. I've immediately spotted the decreased network throughput - I have 500 Mbps link and what I am able to obtain is 60-90 Mbps using a cable connection. But the thing is that the network performance degrades gradually...

I've been investigating this case since, and already had three Windows reinstalls with no effect. The whole situation is quite unusual - the network performance degrades over (short) time - I'm initially able to reach around 450 Mbps, while in 5 minutes I can get only 200 Mbps, and finally after say 10 minutes the speed stabilizes at 60-90 Mbps and won't improve.

Interestingly enough, resetting a network adapter or re-plugging it (I use Ethernet USB adapter) it to another USB port helps for a few minutes. But soon I'm observing exactly the same performance degradation.

The craziest thing is that using WiFi connection I can get over 200 Mbps!

It's surprising, as I had no network problems with my previous installation, and at this moment I'm using all the same drivers (I haven't had the chance to install any software yet). Even more surprising 2 other computers I have at home don't suffer from this and are able to achieve full throughput. So it's clearly this particular device issue.

What I've tried so far:

• using different Realtek drivers (and also trying to use only Windows-provided drivers),
• using different network cables (all cat6),
• using different Ethernet USB adapters (3 tested),
• tweaking adapter settings (e.g turning off offloads, power saving etc.),
• tweaking Windows network parameters (auto-tuning, QoS etc.).

I'd suspect some hardware failure, but in that case there wouldn't be any network throughput at all. I suspect it's Windows that somehow limits the network performance, but I've run out of ideas what to review.

Anyone has similar problem and possibly can advise?