Are you using the original charger with the iPad? Is your USB Cable also in good shape (not frayed, turtle necking, etc)?
Part of this has to do with the iPad's battery. It is a very large battery, designed to run the tablet for a heavy day of usage, which requires time to charge. Usually, 1-2 hours. The 2 Amp charger Apple provides is the correct charger for the iPad. Anything more, and the iPad will not charge any faster since it is designed to pull only 2 Amps of power.
Apple has not incorporated Fast Charging into their devices due to both design and longevity concerns. Fast Charging a battery will damage the battery and can lead to pre-mature failure. Fast charging also generates extra heat, something that is also bad for the battery.