Why is charging very slow

Phone charging speed is a function of how much power P is applied.

In days of old, phones had small batteries and chargers had 0.5mA output @ 5V.

Today, with large battery capacity, phones need lots of power. P=EI.

QC3 fast charging works when the phone talks to a QC3 compatible charger and asks for a higher E.

A higher E increases power. A higher power P increase charging speed.

If it is not QC3 compatible the charger must be capable of outputting a high I (Amps) 3A @ 5V.

So, is your phone QC3 compatible? Is the charger QC3 compatible? Does your cable support QC3 charging?

If it fast charged b4, what changed? If nothing changed, then your charger, cable, or phone has a problem.

Process of elimination troubleshooting.