Although there could be many answers to this question, one reason could be Spyware and/or Malware.  Each of these can download as you are surfing through the internet.  When these files are not deleted and left in your computer can gradually take up internet bandwidth as well as processor speed eventually leaving your computer virtually unusable.
A second reason is your computer may not have enough RAM memory to handle the intense graphics that the internet contains.  Graphics on the internet can easily use up all your RAM memory if your computer does not have a sufficient amount.   When this happens your computer will drastically slow down.