What's interesting to note about this phenomenon is it's not just a property of Slinkies. It's a property of all objects. You can have a really long steel rod and when you let go of the top, the top really starts accelerating down first, and the bottom second. It takes time for that relaxation to travel through any material. You need that compression wave to basically pass through the entire object. A slinky just makes that nice and visible for us.