It's all down to the amps of the charger. The kindle charges slowly on a computer's usb because the usb standard only allows for about half an amp through each port. The wall charger should be around one amp so it charges faster. The fire's charger might be higher allowing it to charge faster. I wouldn't risk a charger above 1 - 1.5 amps on the kindle though or you might fry it.
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