Given a high-throughput network and only 1 or 2 hops between any system, why would the "Securing remote connection" stage be so slow?

Sometimes it looks like the other system is asleep but ethernet power saving is off and it can never fall to sleep because Power Options are set to "High Performance".

What could cause this process to be slow if all network obstacles were removed and systems definitely not sleeping or working on anything?

Also, the domain controller is certainly not overburdened and uses less than 4% of CPU during peak hours.

[RDP connection window](https://i.stack.imgur.com/SDzCV.png)