My xiaomi portable fast charger clearly charges my girlfriend's iPhone faster than my high end android device. Doesn't matter which port, which cable or which android phone. It starts by charging fast but as soon as I plug her phone the fast charging speeds go straight to hers and mine just chargers very very slowly.
I've got the impression that this may be on purpose or at least has some clear explanation.
Does the charger display how quickly it is charging? I assume her iPhone's battery might just have a lower capacity as iPhone usually have smaller batteries than comparable Android devices.