Worst modem connection can be the reason for slowed internet working. For ages I had been using a Linksys modem that I thought it was working good. I was having frequent internet drop outs and blaming them on my ISP. Finally I jumped to a new NETGEAR modem and speeds went up and the drop outs stopped.
There are infinite reasons why your Internet might not be performing as you expect it to. Here are the few common reason for the following: