I checked the logs and a new definition file was loaded at 9:59p and a Defwatch Quickscan started at 10:00p and ended at 10:21p. There's also an event "Reputation check timed out" recorded at 10:21p.
Of note, the period of activity which I captured using Process Monitor runs from 10:22-10:26p. My computer had been running very slowly for at least 10 or so minutes prior to this (I can't recall the exact period of time).
In response to some of the other questions asked, I have SEP installed on my computer which is the only one it runs on, so I suppose my computer is the only "client" (I think). I've upgraded to SEP 12.1.4013.4013 and will continue to monitor the CPU and memory usage.
Several months ago I had been having problems with the Defwatch Quickscan seeming to run and slow my computer every time new definitions were updated. I changed a setting in the registry to disable this, which seemed to fix the issue, at least until now. I would prefer if there's a way for me to have the scans run only when I schedule them rather than at whatever time new definitions happen to be loaded. Is there a way I can do this?