The Victory Bonus is key to progress through the map at good speed. The Victory Points you earn are based on the current Skill Rank of your hero and are then multiplied with the Victory Bonus. A hero with a high Skill Rank in combination with a high Victory Bonus can earn insane amounts of Victory Points.
If a hero reaches a new Skill Rank for the first time, your Victory Bonus charges. This is shown to you in a full screen animation after winning a fight.
Every time you win a fight, the Victory Bonus increases the Victory Points earned. If this happens, your Victory Bonus is slowly consumed and might decrease. The charge of your Victory Bonus therefore is temporary.
Every hero charges the Victory Bonus individually. In case you drop back to x1 and your progress gets slow, try to switch heroes and maintain your momentum. You can always review your current Victory Bonus on top of your hero selection.