Are there any other users in your home using your internet connection? Too many users browsing the internet or streaming movies or other videos can slowdown your internet connection.
Also make sure no one else is connected to your wifi without your knowledge such as a neighbor.
Someone could have hacked into your wifi or be using it if you didn't set a password on it and could be doing streaming or other activities that could be slowing down your connection.
Another possibility is that too many other people in your town are streaming movies or any other videos and it could be causing a bottleneck with your internet connection.