First of all try clearing cookies,browser history and cache,avoid running electronic devices at once,upgrade your modem or router to a newer model and have a periodic check in your modem ventilation, or do you use P2P (Peer to Peer) file sharing programs? If so then try turning the file sharing program off, as file sharing programs use up allot of bandwidth some broadband providers limit the speed you can use the internet if you are using file sharing programs.
If you turn off the file sharing program your broadband speed goes back to normal .I faced the same problem ,after doing this i checked my speed from http://www.scanmyspeed.com apparently my internet speed is faster than before. If these doesn't apply ,there is a chance you may have a virus on your computer. A computer virus is likely to not just slow your broadband and Internet down but your computer too.
If you find that not only is your broadband running slower but you notice that your computer is a bit more sluggish or notice that you have corrupt files or programs are not opening then you may well have a virus.Try to overcome this,your internet speed ll automatically rise up.