Actually, assuming the Hummer is correct in its estimation, only 18.79% of the energy cannot be accounted for, and might be heat...?

Given 49pct charge on 250KWh battery, we have 122.5 KWh to charge. Standard 120v outlet has a max utilization of 1500W continuous, regulated by NEC, so the charger maxes out at that; this gives us 1.5KWh/h charging. IEEE has done studies EV chargers and found on average Level 1 charging is 83.8% efficient without heating present. This is not specific to this vehicle, so it's a rough number. Given 83.8% efficiency, our charging drops to 1.257KWh/h. At this rate, we have 122.5KWh / 1.257KWh/h, giving 97.45h. This is 4.06 days. Assuming that the video is roughly 1030AM on Sunday (he says Sunday morning) and we know the charge finish was projected as 1030AM Friday, we can roughly assume a 5 day charge period. 4.06 days being the optimal estimated charge period, and 5 being estimated gives us an estimate that is only 82.12% as fast as we expect. That gives us the unexplained 18.79% slower than expected charging.

I agree that in extreme cold Teslas and such will use heat to keep the batteries warm enough to charge, but at least if we trust the Hummer computer, it isn't cold enough to significantly heat the batteries, if at all. The unexplained slowness could be heating, or an incorrect estimate about the Hummers onboard inverter charging efficiency, or the Hummers ancillary charging equipment like cooling pumps pulling more power than the average car in the IEEE study. OR it could be as simple as the Hummer being wrong about 1030 on Friday.

My 2 cents