The Paris metro is really nice, but bicycling would be faster if the traffic lights etc were friendly. It's a nice dense city where you could go far in 20 minutes on a bicycle.

On the metro it's easy to waste 20 minutes just on the extras — walking to/from the station, inside the stations, waiting for the trains. Each of these things takes just a few minutes because the stations are close and the trains run often, but few+few+few so easily =30, particularly if you have to change trains.