GT works on a lobby system that relies on every single persons connection in that room.
So, you can have an amazing connection and still have problems due to other users.
Download speed is not important unless you're under 1mbps or something silly, ping/latency is the thing you need to watch out for with online gaming.
That is, the time it takes for data to go to another person and return back to you, and geographically this will take longer and sometimes certain data will not make it back to you, which creates the choppy lag that you see, and sometimes even full on disconnections.
Although, disconnects have been more prominent lately and it seems to be entire lobbies that disconnect not just one person. That's no good for a popular online game.
You can test "Ping" times by going on google and searching pingtest, but that of course is only testing it against a random server that is probably close to you, there's no way to tell what your ping time is against a person on GT.