So changing the priority on explorer.exe really didn't accomplish anything so I snoozed BOINC and discovered TerrariaServer was taking 12.5%. I told them to shutdown and they refused. I ended up killing all three instances of Terraria Server and restarting them. BOINC is running again at 100% and Windows is responsive. And now I remember why TerrariaServer did what it did: by default it runs at high priority. That one process taking up one logical processor was stealing all of the processor's attention. I really, really, REALLY wish they'd patch TerrariaServer to run at normal priority. Having a game server run high priority makes about as much sense as a swimming suit at the South Pole in the height of winter.