USB, USB 1, USB 2, and USB 3? Each version upgraded the standard for faster data transfer and power capabilities among other features. In 2007 a new rule further defined different types of USB ports, specifically the addition of a "dedicated charging port." The charging port can supply more power than a standard or charging "downstream port" and thus charge your USB device faster.
In the USB 1.0 and 2.0 specs, a standard downstream port is capable of delivering up to 500mA (0.5A); in USB 3.0, it moves up to 900mA (0.9A). The charging downstream and dedicated charging ports provide up to 1500mA (1.5A). - ExtremeTech
If you're charging your phone through your computer, you're probably just using a standard or charging downstream port, limited to the slower power ratings of the specification. Switch to a dedicated wall charger and check to make sure it's a high powered charger so that you're not simply drawing a charge at a lower power rating. If you insist on charging through your computer (as you sync, for example), you might also be throttling your charging speed if you're connecting your phone via a USB hub. Many USB hubs aren't rated for very many amps per USB port, so check the power rating or switch to a direct connection. We upgraded our chargers around the home and the difference in speed is astounding. It's well worth the nominal upgrade. MORE USB AND PHONE CHARGING ON APARTMENT THERAPY • How to Never Find Yourself With A Drained Cell Phone • Charging In The Dark: Right Side Up For Cell Chargers • How To: Upgrade a Wall Outlet to USB Functionality • RCA's "Best of Innovation" USB Wall Plate Charger • Outlet Management: 3 Space Saving Socket Chargers (Images: Thomas Pajot/Shutterstock)