Thus is all true about the second car getting less, but 20 miles in 30 minutes is still much slower than I would expect even if you are the second car. The only factor that I can think of is that you also get less if you are near full to start with. I wouldn't expect this to be your typical experience. To get 20 miles in 30 minutes, you would have been getting an average of 12 kw. After 5-10 minutes, the other car should have been below 90kw, and at least 30kw should have been available for you for the next 20 minutes, which in itself should have been over 30 miles.
it may have been a bad connection, your car, the charger, or something else. The only way to know for sure would have been to try plugging it in again and then eventually trying anger charger.