Why is my internet speed so slow all of a sudden

what type of internet connection do you have?

Without a lot more information on connection type, speed you pay for, speedtest.net reports etc, all people can do is guess at your question.

If you have cable inet then most likely it just drops during those times as it's when the most bandwidth is used and cable is split between everyone on the line. Basically there isn't enough bandwidth to give everyone full speed if everyone is using it at the same time.