[Bigriver]

I used the term “label” too loosely. In the screen in the car you can see if the charger is a 250, 150 or 120. On the SC stalls are the 1A, 1B etc labels. Yes, most stalls are labeled with a number regardless of what version SC they are.

 

[Bigriver]

I just looked up that supercharger location on Rt 1 and it is a 150 kW, so the SC stall that you were at was paired with another car. As long as a car was in the paired location, it was reducing your charging speed.

 

[Bigriver]

I also wonder if those of us on Free Supercharging get penalized during supercharging.

No, free supercharging has no effect on how fast your car charges.

got a "45 minutes to continue trip" message

How full was it charging the battery to? This sounds like it was going up to at least 90%. It is rarely time efficient to charge to that high

Perhaps it was because so many stalls were in use that my charge speed was low. But as I watched my charging speed (in miles per hour) as one vehicle after the other finish charging and pull away, and SAW IT GO DOWN??

The 250 kW superchargers, the newest ones, have no dependency with whether other stalls are being used. Older superchargers that are labeled as 150 kW or 120 kW have two stalls that are paired and share the rate. So for those, your charging speed will be cut in half if a car is at the paired location (which is usually right next to you, but not always. Look for the 1A/1B labels for a paired set.)

Your charging rate is very dependent on the state of charge of your battery as Garsh shows above. It will charge at the highest rate when the battery is near empty and the slowest rate when it is near full. So that is perfectly normal that your charging rate went down and did not have anything to do with the other cars near you.

You said the car had preconditioned for 15 to 20 minutes. That is not very long. Depending on the outside temperature, yes, you could have still had slow charging speeds.