I have several USB chargers for my mobile phone. I’ve noticed that one will charge my phone quickly – like in an hour if it’s really dead – while another will take several hours. And connecting a USB cable between my phone and laptop will also charge it, but that seems slowest of all! What gives?
Two things are at play here: how much power your charger can supply, and how much power your phone is using while it’s being charged.
I’ll warn you: for the first, at least, you’re going to need a magnifying glass, or at least extremely good eyesight.