Like any large transport infrastructure, the Internet is composed of many crossings and junctions, each with its own capacity and speed limit. Sometimes we see slowdowns in traffic. In same way that you should stay in your car while the traffic light lets other drivers, the data sent to your computer to wait while other data passes before reaching you.
You logically more likely to experience a slowdown in traffic during the hours when attendance is highest. Similarly, traffic conditions can affect your speed and depend on the geographical situation of the consulting site.