The charging rate is very low when there are many Teslas charging at a station so sometimes it could take 3-4x longer than usual when at a busy location. I'd expect to be charged the same amount in those cases.
This seems like another notch on the long list of hidden caveats with a Tesla. Other examples being constant surveillance/telemetry and the thermal battery throttling (0-60 in 2.xx seconds but good luck maintaining that!)
They do have a disclaimer on charging rate: "Charging from 10% to 80% is quick and typically provides ample range to travel between most Superchargers. Charging from 80% to 100% doubles the charge time because the car must reduce current to top off cells. Actual charge times may vary."
Capacity of the substation or the cable linking the supercharger to it.