Why does take so long for my iPad Air to charge?

That's not unreasonable at all.

it takes about 10 hours to drain it, so not unreasonable that it takes a couple of hours or more to charge it

That's pretty good charging time.

My new iPad Pro takes around 4 hours from 0 charge to 100% charge.

So, your iPad's charging time is pretty good.

Most iPads will take bewteen 3-4 hours to fully charge when completely drained.