From Estfor Kingdom

There are several boosts that can be bought via the in game shop, via third party marketplaces, or by contributing to the Wishing Well. Contributing to the Wishing Well will bestow +XP% gifts once contributions hit certain thresholds, both globally and for your clan (+10% to combat/non-combat/all actions depending on the tier).

The maximum boost currently attainable is therefore +20% from a skill/combat boost, +10% from winning the Wishing Well, +10% from a global boost, +10% from a clan boost, plus any native hero XP boosts.

Combat Boost Gives +20% to combat XP gained for 24h.
XP Boost Gives +10% to all XP gained for 24h.
Gathering Boost Gives +10% in number of items gained from skills for 24 hours.
Skill Boost Gives +20% to non-combat stat XP gained for 24 hours.
Wishing Well Contribution Boost Gives +5% to all XP gained for 12 hours after contributing (contributing again in a single day does not give more boosts).
Wishing Well Wish Granted Boost Gives +10% to all XP gained for 24 hours after having a wish granted (after having contributed the previous day).