The neighborhoods and downtown in Boston can be nice and walkable. But if you want to get around to places around the city, the Boston T has an average speed of about 10 mph [1]. A decent subway system can take you to places at more like 30 mph. NYC subway does about 15 mph [2]. D.C. metro does 30 mph [3].
(Not talking about the train top speed, but the average speed, including stops and acceleration, along the whole line.)
No doubt the T could be better, but comparing the green line is very misleading. It's an above ground street trolley for a lot of its route, and it's much older than any of those other systems. The other Boston subway lines are proper light rail systems that move much faster.
The red line e.g. runs at 20 mph[1]. The problem with the red line are the age of the rolling stock and signal equipment and their tendency to catch on fire.
The red line e.g. runs at 20 mph[1]. The problem with the red line are the age of the rolling stock and signal equipment and their tendency to catch on fire.
[1]: http://www.bostonologist.org/feed/2015/1/29/speed