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The kW charge rate is pretty close to instantaneous - it may be an average of a small window for smoothing purposes, but the window is measured in seconds not over the entire session.
I certainly agree that is the case in Bjorn's video. At one hour 84 kWh were added to the battery (implying an 84 kW average charge rate) while the kW rate is 43.
I'll have to go back to my phone photos of the Model 3 to see if it is the same:
I started charging at 8:50
Ended at 9:14
Charge time: 24 minutes
Range at 8:50: 121
Range at 9:14: 249
Range added: 128 miles = (128/310)*78 = 32 kWh
So average charge rate was 32/(24/60) = 80 kW
The reported charge at 9:14 was 50 kW.