If a target was exhausted (with 2 levels or more) and their speed halved and then was subjected to an effect which reduces speed by a flat amount like minus 10 ft from Ray of Frost, what would be the order these modifiers are applied to their speed?
The closest analog appears to be in Damage Resistance and Vulnerability:
25 damage is first reduced by 5 and then halved, so the creature takes 10 damage.
(Player's Handbook p. 197)
If this convention is followed, a creature with 30 ft base speed would be reduced to
- 10 ft [(30 – 10)/2] with the Ray of Frost example,
or - 5 ft (30/2 – 10) if in the reverse order.
In case where order matters, in what order are speed modifiers applied?
Best Answer
Effects are applied in temporal order
In the damage case, multiple effects have to be considered which already apply to the combatants before.
Contrary to this, the modifiers to speed can be applied in the order they take effect. In your specific example, the person is exhausted, so they don't have a speed of 30 feet, it's already 15 feet. So this speed is further reduced by 10 feet.