Do you actually get the speeds offered?
Regarding the speed, this is affected in two main ways, the quality of the signal and the contention for the resource.
You may have a poor signal (and therefore a lower speed), but if few others are using the resource it may still be acceptable. You might have a great connection, but the throughput could be limited by contention if a lot of people are utilising the same link.
It's also possible that you could have the worst of both.
SD