Power bank capacity is printed at the cell's own voltage, but charging a phone requires converting to a higher voltage. That conversion loses energy as heat, so real-world delivery is typically two-thirds of the printed figure.
Fast charging increases the loss further. Higher power means more heat, so a bank that fills a phone twice at slow speed may manage one and a half charges at maximum rate.
Self-discharge matters for emergency use. A bank left in a drawer for months will be partly empty when you need it, and repeated deep discharge shortens its life.
Safety is worth attention: swelling, heat during idle standing, or damage after a drop mean the bank should be recycled, not used.