I always carry a proper AC charger onboard - not only is the AC charger quicker, but also the electricity flow from some of these stand-alone USB ports may not be regulated to the same standard - you don't want a dodgy USB port to fry your phone or tablet!

If your phone has USB-C, it also means it very likely has Quick Charge technology, in which case the phone will literally charge over twice as fast using the proper Quick Charge charger rather than standard USB port (and in case of the inflight USB ports, a low-output one at that).