A general rule of thumb for determining whether or not you have a good broadband connection is whether or not you are getting at least 80% of the maximum possible speed you could. There is a built in 'loss' due to overhead information that must be transmitted and a few other factors that immediately eat up about 13% of your bandwidth (speed). That means that the absolute best in a perfect setup would be 87% of your theoretical maximum. With other factors thrown in, the 80% rule of thumb was developed. Now, how to apply it.
As one example, lets assume you have a 1.5/256 DSL connection. This means that the best you can ever do is to get 1500kbps (kilobits per second) as a download speed and 256kpbs upload. But at least 13% of that is going away for the overhead. 80% of 1500 is 1200kbps, and 80% of 256 is 204.8kbps. So if you have a 1.5/256 line and are getting speds of 1200/204 or better, then you have a 'good' connection.
If you are getting less than these numbers, the DSLR Tweaks forum is a good place to go to start optimizing the portion of the system that can help with these numbers.
A few things to keep in mind: 1) You can only 'tweak' to improve your download speed and hopefully your upload speed will rise along with it; 2) It is hard to apply the 80% rule to an uncapped Cable connection where no upper limit for speed is stated; 3) no 'tweaks' are available to improve Ping (latency) times - that is a separate issue.