aramis erak
Legend
Eh. It's not too far off for 3e and 4e, in effect. In 3e you had to take a full-round action, provoke AoOs, and it just made it a critical that you got to save against: the DC was ridiculous, but frankly the number of immunities in 3e meant that your chances of CdGing effectively was much less (both for landing an initial condition and for the thing not being immune to critical hits, death effects, etc)
In 4E, you had to do half the target's hp in one hit to kill it. You're not going to do that even with a crit in 5e.
Since we're talking about ~20th level you can assume several crits (iterative attacks) instead.
Every edition is different, but there's a reason I mentioned an assassin - hold person is low level, and you should kill any humanoid (especially a rogue-like one) with a round of autocrits. As opposed to hold monster which might find a creature that has too many hp to compete.
They failed to kill a sleeping kobold with a crit... the rogue didn't, but the mage did. (He did drop him 1 point shy of dead. With a fire bolt crit.)
I saw more crits with damage under 5 last night than I thought possible. Of the 6 crits I rolled last night, only one exceeded 5 points. (Dagger crits don't do much.)
And some of the human NPCs are into the 20 HP range; in my group of 3rd levels, 3 of them generate 6d6 on crits, . The two rogues, using their sneak attack (Advantage vs prone gives 2d6 more damage; crit doubles the dice thrown, wepaons are either 1d6 or 1d8 base) get either 2d8+4d6 or 6d6 on a crit. The fighter with greatsword does 2d6 base, doubled on crit, +2d6 for being a half-orc, for 6d6. But they aren't going to kill a 50-some hp knight who's held. It will take, on average, 3 hits to zero said knight, and another 2 to ensure he's dead. In a large combat, that can be a lot of time taken aside.