There are many better ways of training someone to go faster, and weighing down people’s legs would be at or near the bottom of the list.
Sometimes weighted vests are used for specific drills, but in order to get faster, you need to run fast, and you can’t do this by weighing down certain parts of your leg. For one thing, you’d mess up your mechanics…I’m sure other people can go on and on about how this is a really bad idea.