In downtown Boston, T-Mobile's carriers are actually pretty overloaded, at least relative to areas around downtown.
For example, where the Boston bombings took place, that area is notoriously overloaded (it's the area where I work, so I am very familiar). Sprint for the past 5+ years has been completely burdened and has trouble breaking 100kbps/sec over 3G. T-Mobile is also burdened but I'll typically get over 1000kbps (when my nexus 4 latches onto T-Mobile's unburdened PCS band, performance is about 8Mbit in the city). When I get about a mile outside of Boston, speeds on Sprint's 4g LTE network average about 8Mbit while T-Mobile's DC-HSPA+ average 14-18mbit. It's scary because Sprint's LTE network is fairly new and I'm watching the speeds flip around where uploads are faster than downloads... which isn't good at all.
All carriers seem to be pretty slow in Boston proper. Too many people, not enough cells, not enough spectrum. Go 1-2 miles around the city and things fly.