My mistake, I thought this was about batteries, not a USB battery pack. A USB pack is going to charge at 1.5 to 2.0 amps, limited (as mentioned above) by the internal circuitry. There is no way to speed it up, nor should you try.
What you should do is get a USB ammeter to find out what rate the pack is actually charging at, so you can try different chargers and cables to get as close to 2A as possible.
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