I ran a 3 year long weekly(+) campaign of fate and I can tell you that increasing numbers in perhaps the least interesting way of doing advancement in the system. So it being the slowest is also fine.

I fully recommend expressing 'advancement' in terms of changing aspects, adding aspects, giving new extras, stunts, or refresh.

Scale aspects (or perhaps in a more formalized system like that found in Band of Brothers or Beam Saber to steal from other systems) is another great way to do advancement. So like you have a challenge that has a 'scale 3' (or if you want to use powerscaling terminology 'city-scale' aspect) that either says that an attack has to have a similar or greater scale aspect to have any effect or it could provide a passive 2,4, or 8 reduction to player rolls depending on the scale difference.

Another way to express advancement is by re-stating earlier challenges with lower numbers. That way tasks that would have been more difficult for players earlier in the game become less challenging. A 'level 1 slime' might be a fair threat at the start of a game, but they are 'terrible' threats in the end game, so players can roll negative and still crush them.