To answer that question, we first have to understand the vehicle’s battery. When most people imagine a car battery, they might imagine one big block sitting inside the car. In reality, inside a “battery pack” are hundreds — and often thousands — of smaller “battery cells.” (The Tesla Model S has up to 7,104 battery cells!) As a result, when a battery charges, those thousands of cells are actually what’s being charged.
A helpful analogy might be to imagine sitting in a movie theater. When the theater is empty, it’s easy to find a seat right away. But as the theater fills up, we have to take a few moments to find a seat — and climb over people (without knocking over their popcorn). That’s what happens with battery cells at the molecular level. When the battery cells are nearly empty, it’s easy to “find a seat” to charge. But as the battery cells fill up, it takes more time to find (and navigate) the empty cells. Generally, above 80% full is when it’s hardest for electrons to find a seat in your battery’s movie theater.
NOTE: Your charging speed will slow down throughout the course of your charge. And every vehicle decides that “slow down rate” differently. Every manufacturer determines this in order to keep your vehicle’s battery healthy and increase longevity.