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.

[1]: http://www.bostonologist.org/feed/2015/1/29/speed