21 miles from the runway, or 15 behind the Cessna. Now consider, when I'm working finals, my airspace only extends 22 miles from the runway. If I have downwinds extending beyond 20 miles, I'm supposed to start making pointouts to my fellow "feeder" controllers, and I'm in the protected airspace for arrivals and departures off two satellite fields.
So, that's why the controller is hammering you to keep the speed up in that Cessna, because just an extra 20 kts on your part means about 4 miles shaved off the downwind for the controller. Also, turning the Cessna on at the app gate, rather than two miles outside of it, saves at least twice that (4 miles) for the Boeing behind. (unless it's a light B757, because they fly final as slow as Cessnas anyway....)