And monsters grow up on hp per CR in a much exponentially way. High level encounters on 5e can take dozens of rounds.
Many DM's also complain that is hard to challenge a party on higher levels. Because nothing is a threat to theyr near infinite hp pool. I love high level but for 5e, we should focus more on mid level.
I'm curious where you're getting this idea from...
A mid to high level fighter can easily put out upwards of 80 damage a round without burning any resources; a 6th level disintegrate does an average of 70 damage.
Compared to this, even demon princes and deities don't really get much above 300 hit points; There are literally 7 sub-CR20 creatures with more than 300 hit points, and most of them are end-of-module bosses.
Even if you push the bracket as far as it will go, with the most recent high cr books, there are only 14 creatures, ranging from CR 22-28 that break 400Hp, and again, they are almost all end-of-module major bosses.
A party of 4 that are at a level to challenge such creatures without getting chunked in seconds are going to have damage outputs that might see them doing ~200 damage a round as a group, or more.
None of these battles will EVER take more than three or four rounds at most, one way or the other.
Edi: I'll add to that, your wizard with 14Con will average about 110Hp at level 20; That's barely more than one round of damage for most of the above-mentioned creatures, not including legendary actions/reactions - grab a stock standard ancient Blue at that tier; its breath weapon deals average 88 damage. A Marut deals 120 damage to a single target a turn if it wants.
High tier battles are fast and brutal in 5e, the exact opposite of slow and drawn out.
