John Wick kills (almost) everyone with one shot. The only times he doesn’t are because of an epic cinematic duel meant to draw you into the fight. Your tabletop combat could benefit from this, too. I’m not going to say “HP is meaningless,” because you want the players at your table to feel that they can gauge the toughness of an enemy. The Challenge Rating system is far from perfect, but its a good marker. The players generally want to win, but want to win while feeling they’ve accomplished something. Maybe they’re critting or maybe they’ve got the enemy tactics figured out. You’d hate to see the combat end after two rounds, but maybe its been going on 30 minutes because some players are slow to decide on an action. There are many considerations to take when running a combat session as a DM, but the general idea is still for everyone to have fun. Combat is meant to make them think quickly and adapt to the changing battlefield circumstances. Nobody wants to build up 10 sessions towards a fight with a dragon to have it go down in 2 rounds (or 15 for that matter). The simplest approach to this is to just end it when it makes sense. Maybe the casters have burned through some spell slots, the party has taken some damage, and you can tell the players are checking the enemy for its condition to see they’re close. Perhaps that fighter pulls off a great move and then lands a big crit. Generally tracking HP along the way is fine, but if that dragon is left with 4 HP but the attack was so fantastic and a climax of the fight — end it. If that crit was the first round and your min-maxer is over there planning out their 5-part action and the stat sheet says the monster was gonna go down — maybe just ignore its HP pool until the last person gets a shot.
Identifying when to end combat or when its thematically appropriate is difficult and subjective. Some players may metagame a bit and know that zombie should only have 20HP, but you just blasted it for 30, so you can’t fully ignore those mechanics. In the same way that a DM with a screen may fudge some rolls for the sake of the roleplay or higher stakes in combat, take the approach with HP pools or enemy counts. What I personally do is just use the stock HP for the monster. If it works out and it flows in combat where you use the original pool, great. If its going too fast, just tack on another 20HP or whatever after they take damage. Homebrew a little regeneration ability or something if you want. If its going too slow, just chop off another 20 from the enemy to progress things. Maybe the big baddie went down, but there are a few henchmen left. Its not always fun to just mop them up after and mow them down with the tension gone. You can have them forfeit or run away to signal victory. Combat in your sessions is ultimately about telling part of the story and involving the party, so take considerations for the story that everyone is helping to tell and make those battles that everyone’s prepped for feel rewarding with high stakes and high tension.