It's the ancient print spooler, made worse by Windows Update (who would have thought?).

Last night, my project suddenly slowed down and stalled, maybe four times in a short period. Poked around Task Manager, and sure enough, the Spooler Subsystem app was hogging 25% CPU and almost 1 GB RAM. Turned it off, and didn't have another problem.

Internet is full of reports of Windows Update causing this, others blame it on old files hanging out in your Printer folders, that spooler is trying to send persistently.

Whatever the case (search!), this simple step made me a happy camper. It's cheap to try.