I just noticed that the charging speed on my display shows the Charge Current as 48 A ( Model S - bottom left corner of the charging window). If this were using DC charging, wouldn't it have a much higher value than 48 A or a different value completely? I haven't been to a different charger for some time, so I'm not sure what to compare this to. But my feeling is that it is doing Supercharging via AC instead of DC which is why I'm not hitting a higher speed.
No, Supercharging is always DC, and it ignores the charge current setting on the charging screen (that's the maximum current your car would draw from an AC charger).
Bruce.