By simplest idea and the closest to something they know. For example, I liken the loading speed of a website to the waiting time at a traffic light or at a checkout counter in a store. If the road is crowded , the wait is longer, and vice versa. I also use visual examples such as illustrations or short videos that explain how loading speed affects the user experience, such as how visitors feel when they wait a long time to enter a certain site. I like to explain things in a practical way, explaining to them that every second of delay could mean losing a visitor or potential customer. Sometimes I share success stories or practical experiences from my previous projects, which makes them understand the importance of the subject more.

Try to do it as easily as possible, like you explain it to your kids. With pictures from global to small things if they are interested in discussing it in detail. 2 computers (client and server with some apps) with the pipe (pipe then will be a cloud of internet providers), show how data flows...