I was in Bristol yesterday and needed a charge to get home. Both bays at Cribbs Causeway, apparently spare. However, on arrival, 5 mins later, a Model X was charging. I started to charge my 2016 S in the remaining bay. 15kw charge out of 130kw! I thought it might recalibrate but it did not. I even stopped and restarted the charging, but to no avail. That seems totally unbalanced and pointless for the 2nd vehicle. 2/3 to 1/3 I could understand but 90% to 10% seems faulty and the word supercharger registered invalid!
This is pretty well-known: First car in paired stalls gets 100% of their charge rate & 2nd car gets whatever is left of combined total. Lesson: select your charge bay based on not being paired with an occupied stall if charge rate is important.
It starts at 75%/25% but there was evidently a problem with the charger that resulted in lower output. Why is this in the Roadster 2020 forum?
It's also possible the OPs battery already had a high state of charge, limiting the kW the Supercharger would provide.
Assuming your battery was low enough that the car was capable of accepting a faster charge, then the hardware wasn't fully working properly. The second car to plug in should be getting up to ~30 to 36 kW if the first car is charging with a low battery. Once the first car's rate tapers to below 72 kW, the second car's rate will increase.
That is normal elsewhere. I was low. That is why I wondered if the big X was favoured. Sounds like equipment not doing its stuff . Thanks.