You can override the Ability Play Rate / Cooldown and mark the Cooldown to be updated in real time (so, if you cast a normal fireball, but then buff yourself - you can retroactively decrease the in-progress cooldown). So when you cast Haste, just put a tag on the Player “Effect.Haste” or something like that, and in the Play Rate you can check for that flag and modify the play rate as you wish.