FWIW, the charging rate is first bounded by the device involved and second, by what the vehicle wants or can accept at the time. To explain, the EVSEs are designed for some maximum amperage. They output a signal that tells the car how much that is so that the car will not try to draw too much, and overload the supply circuit. But, power = volts * amps, and the supply voltage is not always consistent. With a few exceptions, the i3 can accept a maximum of 7400W. Solving that with the limitation of say that 30A, would mean 7400/30=246.67vac input. If you have, say 220vac input, limited to 30A, that would be 220*30 = 6,600W. FWIW, in the USA, a common commercial power level is 208vac, so that 30A device would only provide 208*30=6,240W. Generally, the grid is regulated to provide +/- 10%, so that would adjust the power levels accordingly. In extreme conditions, it can vary more.
If the signal output from the EVSE is off a little, it may be telling the vehicle it has say 29A available, which would limit the amount even more. If the car's response to that was not calibrated, it may not try to draw as much as was available. Ideally, the EVSE would have more power available that the vehicle can accept, then it would always maximize the charging rate, but practical conditions to not always allow that. While my EVSE is limited to 30A (a more common level today is 32A max), because my typical input voltage is fairly high, I usually can max out the i3's capabilities, but even it I couldn't, generally, I've got overnight before I need full charge again.
What might be annoying is if the unit has a charge based on time verses Kw delivered...you'd want it to always max out the vehicle.
Then, the car won't allow that maximum rate all the way up until it is full, or even at the beginning, if the battery pack is either too cold or too hot. It will taper up to the maximum (assuming it needs enough juice), and then taper down as it approaches full.
If the device is announcing an average, verses instantaneous, that can be an issue to, as your peak would likely be much higher.