Ques. why internet speed is slow when many people share internet connection and wifi connection.
Ans. Dear sir consider internet as a highway when to many vehicles on the road the road gets chocked.
So, say when 10 people use an internet link of 10 Mbps link , the first hop ( ie the immediate Switch/Router) is Default programmed in such a way to distribute the 10 Mbps link evenly to all the 10 people. So, each person will get bandwidth of 1 Mbps considering all the people are online at the same time and are using bandwidth at it's fullest.
If you want to priorities all the 10 Mbps bandwidth to one device that can be done by going to the management console of the (Switch/Router) and changing bandwidth distribution priorities accordingly.
Remember the simple rule more people using the same link, lower the speed each device will get.