A power adapter is rated by watts. USB power adapters use about 5 volts and they come with various currents. Watts = volts x amps. For example, a 5 watt USB power supply has 5 volts at 1 amp. A 10 watt USB power supply has 5 volts at 2 amps. A 5 watt (5W at 1A) USB power supply will likely charge a device slower than a 10 watt (10W at 2A) USB power supply. It doesn't matter which company makes the power supply or what label they put on it. What matters is the number of watts. In general, the more watts the faster the charge, but only up to a certain point. The devices are designed to except a maximum amount of current and will not benefit from power supplies that have way too much current. Long story short, if you want faster charging then go with a 10W 2A USB power supply rather than a 5W 1A or less power supply.

If you have a few USB phone chargers, and you likely do, look at the plug portion and try to read the wattage level. The print is often very tiny. If you see 5W or 1A, then that is about what the standard Kindle charger has. If you see 9W or 1.8A that is what the Kindle fast charger uses. For example, my iPad charger is rated at 2.1A.