said, it's most likely that you've not set the charging rate to 12 amps. It defaults to 8 amps in case the house wiring is old/scary, and will sometimes forget and auto-reset back to 8 amps even if you've set it previously.
At 12 amps, at best you'll get 120V * 12A = 1.44 kW per hour, which at 89% charging efficiency translates to 1.28 kW added to the battery, assuming the car isn't using any energy for anything else, like cooling. That would be about 5.1 miles added per hour if you averaged 4 miles/kWh and only 4.5 miles if you only averaged 3.5 miles/kWh. Cooling would eat into that. If it is really hot (necessitating cooling during charging) and your general driving efficiency is low, and your house supply is closer to 110V than 120V, it's conceivable that you might be getting closer to 3 miles/hour of charging even at 12A.
is wrong to say that you're supposed
to plug the provided adapter into 240 volts. This is a trick the community has discovered to double the charging rate from the stock adapter (and determined is safe from disassembling the charger and looking at the components used). You can totally do it (and I have), but it's not something sanctioned by Chevy nor is it needed to get charging at 4 miles/hour for most owners. But if you have a 240V outlet, like a dryer outlet, it's certainly the cheapest way to get a boost in charging speed.