What brings about first and slow charging effect on the phone chargers?

[Stardom22]

There are some phone chargers that function more efficiently and actually does charge phones at relatively higher speeds compared to others. What is it that brings that difference?

 

[eLdavis]

Each phones have their specific chargers, the amount of watts often determines the charging speed. A charger of 120 watts would definitely charger faster than that of 90