Why are people so insistent on a power bank. I think power banks are good to have but to me they're meant for day use when I'm OUT and about and not at a plug. If you are at a plug, then plug in, but as I said earlier, bring your own AC adapter. There's really no reason to be using USB ports. If you charge an iPhone at 5W for 1 hour how much battery do you get? So bring your own adapter, get fast charging and actually make a dent.
What I find funny is I see people plug into ports or use 5W chargers, then use their phone simultaneously at the airport. When you consider data congestion at public places, they're probably getting 10% / hour charging at most, which basically did nothing. Using a fast charger can give you something like 50% over 30 minutes, and even if we cut that down a little to account for active using, you can probably get 30-40% bump charge which is significant (imagine being at the airport on 20% charge and bumping to 50-60%... that's big and can allow you significant time on the plane or later).
Now if you have a power bank you can continue charging on the plane assuming the plane has no plugs (ahem Southwest), but my point is if you can access a wall plug, do so first.