Why is my iPhone 4s so slow?

My iPhone is pretty new I got it about 2 months ago. I use Sprint and I have wi-fi. The problem is that the wi-fi signal is full and the internet is still really slow! My brother and sister have the iPhone too and theirs is really fast. I have no clue what it is they both have the black iPhone and I have the white if that makes any difference.

Sprint iphones are the slowest ones of any carrier.