There are a lot of factors that will affect your Internet speeds. The computer you are using, its processor, how many programs you have opened at the time you are on the Internet and/or whether your computer has been infected with spyware or viruses–these can all effect your Internet speed.
The overall traffic on the Internet can also effect your Internet speed.
The number of devices sharing the same network as well as potentially faulty external devices, such as routers, can affect performance.
Click Here to watch a video on how internet speeds work.