I ruled out the system page file on my Win 10 laptop by disabling it and rebooting. It runs OK with 4GB RAM and no paging. It is still worth a shot to try and troubleshoot though, as all systems are different, and there are probably many possible causes for these symptoms.
My HDD was still busy after booting. But once Windows settles down, everything runs smoothly.
Hardware tests fine, no errors.
So in my case, it appears to be a Windows issue. I'm just going to live with it, or upgrade to an SSD. But since it's just a computer for traveling, and not my DAW, not that concerned about it. Just wanted to share info with the OP, since it sounded very similar to his case.