Is it time to start calling out bad cyclists?

Yes, sit behind, overtake later. You can never tell how fast someone is from how they look.

I look slow whilst stationary on my shopping bike, but am actually awesome. When folk move up to sit outside me at the lights thinking they're quick I very much enjoy moving off at the same rate as them then keeping exactly the same pace on their inside until they give up and tuck in behind.

It's a great game.

I once drafted a rather portly MAMIL on a Cervelo on while I was riding an old steel 'Sport' frame with North Road bars and a huge wooden wine box on the front, which happened to have an entire BMX in it (in pieces). He got so angry at his in ability to shift me off his wheel he actually started shouting at me. Rarely have I had to stop cycling via laughter.