True, I use "2A" loosely as max charging speed, but it actually close to 1.8A due to efficiency loses. Regarding Galaxy Charging app, my experience has been relatively good and I've verified it by actual charging time vs charging percentage of the battery (to determine actual mAh increment versus h time). I know it will not be as accurate as Battery Monitor Widget, but it gives you a very quick result based on some internal current sensing monitor.
There are too many variables in charging equation. The wall charger and its aging (some will have a drop in efficiency over the time, even OEM ones). the cables and how thin/thick the wires (you will need 24AWG wire for a full 3A throughput), micro-usb connector pins and how good they are making a contact, even your wall outlet will make a difference if you have it shared with other wall outlets (from the same power branch) or if you have your wall charger connected directly to wall outlet or through extension chord, etc.
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