How would you institute a "haste" or "slow" spell,

So I'm creating this final fantasy hack as you may have seen in my other thread, anyway, I want to put the time mage in but I'm not quite sure how I want to handle the time magic... rather how I can handle the time magic since there are no "turns". What do you think? I was toying around the idea of them gaining some kind of "haste" point and they can spend it at any point in combat to make a move or something....

HASTE                  Level 5             Ongoing
Choose an ally : he is now really fast. When doing a hack-n slash, on a 7-9 he get to choose one :

• He can menage another hit doing + 1d6 damage.
• He is fast enough to avoid  the ennemy attack.

On a 10 + He can menage + 1d6 damage without exposing himsefl to danger.

SLOW                  Level 5             Ongoing
Choose a target : it is now really slow. When someone does a hack-n slash against it, on a 7-9 he choose one :

• You can menage another hit doing + 1d6 damage.
• The target is too slow to be able to do any damage to you.

On a 10 + you can do + 1d6 damage without exposing yourself to danger.

You could use it quite "Narratively", allowing the player to simply use "time" as a special effect ("The Ork swings his Axe over his head and pushes it down on you, with all his might"-"Waves of magic from my Hand bring the Orc to a halt, I step aside, and just as syncs back with us and his Axe hits the emptiness, I kick him from behind" - "Roll for H&S if you can pull this of fast enough.").

Or what about: "Universal Preparedness: When you prepare your magic in the morning, you step a bit in the future and make yourself useful. roll+Int, 10+:Hold 3, 7-9: Hold 2, Fail: Hold 1 (plus whatever goes wrong). Spend one Hold to Aid or Interfere during the day, whether you're with the characters or not. You can even Aid or Interfere yourself."

Other options could include:
1: Gaining Hold that can be spent to "act immediately".
2: Allowing the caster to use INT or WIS to Defy Danger against physical threats (sort of a time manipulation dodge).
3: Treat "Slow" as a Tag.

Just a few thoughts.

Haste          Level 3          Enchantment
Choose an ally, they gain 1d4+1 speed. They can spend speed 1 for 1 to:
  - Change your position on the battlefield in the blink of an eye
  - Strike someone within reach before anyone can react
  - React to a threat or danger instantly, even if surprised
While they hold speed, they must Defy Danger to do anything that requires patience, care, or stillness.

[Edit]: Not sure how I'd handle slow, except to say something purely fictional like "Choose an enemy. While the spell lasts, they move and react slowly, like they're underwater, and you take -1 ongoing. The spell ends with you dismiss it or cast another spell."

I like Jeremy's suggestions for Haste/Slow the best so far. Whatever you choose, I'd stick with a Hold mechanic for the spell. I wouldn't just make it "deal more damage," that makes it just another offensives spell. Instead, it's like what does it mean to be magically fast? It means getting to use your magic speed in ways others can't. So give 'em hold and say 'When you spend a Speed Hold, describe how your Haste spell has given you the advantage and..."

In terms of time magic, I would keep that strictly narrative, not another combat spell. You're trying to shoehorn in "turn" mechanics where there are none. Ragnarok's "Universal Preparedness" spell is the best for this so far, and it also uses Hold. Hold is a great mechanic for giving players a measure of control over the narrative, as the result of an ability. It's basically like "okay, you can break the rules X number of times. There you go."

In a related comment, there's a reason most games don't mess with time travel. It messes with continuity and player input way too much for an interactive experience. It leads to second-guessing and redoing scenes and madness, madness I tell you...

Thanks for the tips! You guys rock.