It has to do with the output amperage of the charger. I believe the OEM charger is .75 Amps (or 750 mA). Charging though a USB port on a computer is 500 mA so it's even slower. I use my old Palm Pre charger. It is 1 Amp. It charges significantly faster. I wouldn't go higher than 1 Amp. It might burn up your battery or worse.
So look for a charger with a micro USB plug, with a DC output of 5 volts and close to 1 Amp.