What level should this exhaustion-causing spell be?

Sap strength
Casting Time: 1 action
Range: 90 feet
Components: V, S, M (a piece of a jellyfish)
Duration: Instantaneous
Choose a creature that you can see within range. The target gains a level of exhaustion, or its level of exhaustion is increased by one.
At higher levels: You can target one additional creature.

No save!

6 castings of this spell can kill any one target, who is not immune. But it takes 6 rounds, which is longer than most encounters.
One casting is about as useful as the first level Hex. There is no concentration needed, but this does not increase DPR.

I was thinking level 2, considering you need two rounds just to halve the target’s speed.
What level should this spell be?

To avoid pulling opinion-based answers: an ideal answer will reference similar spells (if someone has homebrewed and playtested a similar effect). In the absence of that, answers from experience related to the exhaustion mechanic, effects that grant exhaustion (be it from cursed items or other penalties imposed by the DM), or other effects that impose penalties that are like exhaustion are acceptable as well.