Someone online was telling me earlier that they needed more RAM in their new Windows 10 machine despite already having 16GB in their system. Their RAM use in Task Manager’s Performance tab was in the order of 15GB (even when idle) but the Details tab did not show any one process with more than around 150MB used.
I talked them through running Sysinternals RAM Map. On the screenshot he sent, the Non-Paged Pool histogram portion was massive. Here’s a close-up showing the figures:
This suggested a duff driver and enabled me to do a more meaningful Google search on the issue. There were a number of hits describing the same problem all pointing the finger at the same cause. Here’s one such page.
As on the above, he did indeed have an MSI motherboard with Killer networking. After a driver update from here, the problem went away. Plan B would have been to disable the Network Data Usage Monitoring service with a reghack as suggested by the sites I found.