Maybe your in a Capped package. Meaning your internet bandwidth per month is capped/ have a limit.
Let say 20GB worth of bandwidth. Most Internet service providers do this. And after you have reached your bandwidth limit. 40% of your connection speed will be reduced.
If that is not the problem, maybe you have a virus in your system. Or someone is illegally connected to you, specially if you have wireless network.
Actually the reply of Rahul to clear your temp files. Is somehow not a good idea. (Only do this monthly or annually)
Temp/Temporary files are used by your browser to load things faster, other objects of the sites you visited are being put in your local computer so that the next time you visit that site, you wont need to download those objects again (ex: logo of facebook) and will give you much faster surfing.