If you check the article you will see that every charger has an Amp rating (like: 2 Amperes). This has a direct impact on the charging rate (provided that the device can handle it). Charging a device at 5V@2A is twice as fast as charging it at 5V@1A. An USB 2.0 plug can usually provide 500mA (0.5A), so the USB charging is slower than using the plug.

In real life charging is also limited by the battery temperature for safety reasons. If the battery becomes too hot during charging (because of the environment temperature, poor ventilation or just because the device has a huge consumption while charging) the charging will stop and will start again when the battery cools down. This is why we do not see chargers with bigger Amp rating: the batteries are limited to a current that doesn't get them too hot.