After heavy use, my big iPad has charged very slowly. What can I do besides the Genius Bar? Is there a different wall charger I should use?
iPads do need more current than iPhones, they have bigger batteries, and bigger screens. They consume more power because of this. If you aren't using a charger (block) of the same or higher capacity than the one included with your iPad. Considering this is a Pro model we're talking about, it'll probably have to be a higher voltage charger than a regular iPad charging brick. If you ARE using the charger included with your iPad Pro, consider getting it replaced. It may have given out from a power surge, or just worn out from constant use.

Temporarily, however, you can power off your iPad while charging. I've had a similar problem to this with my Samsung Tab A 7.0, and powering it off allows it to charge. It will probably still charge slow, but it won't lose battery life, and will charge significantly faster than with it powered on.