Stunned vs. Slow

[TriciaOso]

I've been thinking idly about creating a Monk subclass based on the Pratchett History monks. I thought about replacing Stunning Fist with a slow effect.

Slow:


An affected target’s speed is halved, it takes a −2 penalty to AC and Dexterity saving throws, and it can’t use reactions. On its turn, it can use either an action or a bonus action, not both. Regardless of the creature’s abilities or magic items, it can’t make more than one melee or ranged attack during its turn.
If the creature attempts to cast a spell with a casting time of 1 action, roll a d20. On an 11 or higher, the spell doesn’t take effect until the creature’s next turn, and the creature must use its action on that turn to complete the spell. If it can’t, the spell is wasted.

Stunned:

A stunned creature is incapacitated (see the condition), can’t move, and can speak only falteringly.
The creature automatically fails Strength and Dexterity Saving Throws.
Attack Rolls against the creature have advantage.

Stunned seems overall better; is there something I'm missing in terms of balancing the price, or should it indeed be cheaper/more rounds?

[Composer99]

If you're making a subclass, I wouldn't recommend replacing Stunning Fist. Instead, give effects that can manipulate time - haste, slow, and maybe even 1/long rest time stop as the subclass capstone.