It can't charge from 15% to 100% on 20 minutes. For starters, anything over a 1C charge rate will shorten the battery life a lot. So you really want it to take an hour OR MORE.
The battery is 2100mahr. The phone charging electronics pulls AT THE MOST near 800ma. at 5 volts. So that is 2.5 hours or so. PERIOD. It doesn't matter if you have a 1 amp or a 100 amp charger, the phone will pull around 800ma. About 2.5 hours for a full charge.
That is the way it works, and that is the way you want it to work. If they designed it to charge at, or faster then 1 hour, the phone would get very hot, and the battery would have a short life.
It is possible that the BB charger is over 5 volts. Check the voltage on it. If it is over 5, you risk over heating something and damaging it. Don't use anything over 5 volts.