Internet can slow down when more users are using the internet.
For instance if you have WiFi internet and more than one user in your house is using internet service over that WiFi it can cause congestion on your internet line which can cause your internet service to slow down.
Also if a bunch of people in the neighborhood start using more of their internet such as streaming videos, music etc then your internet service can start degrading and slowing down.
The more users the more traffic on the internet service lines which means a slow down of your internet speed.