The sheer number of users in NYC in manhattan would bring the network down a few steps. The available bandwidth of a single sector is proportional to the number of users on that sector. As people get added and are talking/texting/surfing, the available bandwidth for each user is cut.
When you get a really fast transfer, you were probably one of the few on that sector at the time... If you are moving around, unfortunately in the GSM world, those are hard handoffs from sector to sector (and tower to tower at times) so the connection is kinda paused and restarted on the new sector. If you are only on that sector for a short duration and move to another, then you are dealing with another pause and restart...
When you get a really fast transfer, you were probably one of the few on that sector at the time... If you are moving around, unfortunately in the GSM world, those are hard handoffs from sector to sector (and tower to tower at times) so the connection is kinda paused and restarted on the new sector. If you are only on that sector for a short duration and move to another, then you are dealing with another pause and restart...