Very unlikely to be caused by the gas cap. I can’t think of a way it could be.

You’re right. The coolant prevents rust and corrosion and even distilled water does not. Coolant also lubricates the pump better than water.

What kind of storage is in each PC? Are they both high quality SSDs?

Can you test the network speed using iperf?

https://iperf.fr/iperf-download.php

iperf is run from the command line. One PC will be the server and the other will be the client. There are youtube videos on how to use iperf.