Instead of rolling for every creature hit by a spell, can we reasonably use a single additional die for the number of hits?

I will preface this question by stating that the hypothetical situation proposed below will obviously only work as long you have a die with a number of faces equal to the number of enemies.

Scenario

Suppose we have a party of PCs and they encounter 8 identical hostile enemies. Once of the PCs is a wizard, and decides to cast Fireball in such a way that it will hit all 8 enemies at once. Since we don’t roll to-hit with Fireball, all the enemies just make a Dexterity saving throw. These rolls would normally be rolled for each discrete enemy for a total of 8 individual rolls.

Question

Would it be entirely unfair to roll a single d20 and a single d8, and if the d20 roll succeeds on the Dex save, rule that the 1d8 roll is the number of enemies take half (or full) damage, and the rest take full (or half) damage? Conversely, if the Dex save fails, all enemies take full damage.

Thoughts

I’m considering an approach to speeding up dice rolls for large packs of enemies, but I can’t really tell outright if the above scenario is unfair. My first impression is that it might skew based on saves or AC (if we’re rolling to-hit instead of for saves) and number of enemies.

For the example let us consider two sets of outcomes: one for a pack of Kobolds, and one for a group of Bandits.

I know this all essentially boils down to dice math, but I’m not the greatest at it and would appreciate the help evaluating whether this is fair, and what the reasonable thresholds are.