Slowing the rate

Ideally, PCs should Level Up only once between adventures. You want players to see each new move in play before they gain another. If your players often Level Up multiple times between adventures, consider some of the following.

Reset XP to 0 on Level Up. The simplest tweak, but some players might hate it. It also undercuts XP incentives once a PC has enough to Level Up.

Increase the XP requirement. The default requirement is 6 + (2 × level). Increase 6 to a 10 to add an extra session to most levels. Multiply their current level by 3 instead of 2 to really slow the rate of advancement at higher levels.

Adjust the End of Session questions. Be stricter about what counts as a "Yes," or rewrite questions to be harder to achieve. For example, "Did we learn more about the world or its history?" could become "Did we unearth any secrets of the past?"

Let players always Burn Brightly, even if they lack the XP to Level Up. (If you have a Would-be Hero with the Driven background, you'll need to tweak that move.)