So back to the plumber. He was an hour late, so I called and they said they had the appointment booked but "forgot" to send a guy out and did so immediately (nice that it's a small town). He showed up and I showed him the faucet and what I'd done, and then he reaches up and finds a second hidden "flow regulator filter" in the line I didn't see. Unscrews it, blows out a blob of gunk, puts it back on and voila water pressure. Took him all of 10 minutes and that includes the elevator ride up. It took him longer to write up the $110 bill (minus $50 for "forgetting" the appointment) than it took him to fix it.
Well at least I know now where to look when I get low water pressure again.