Again, how?
Presume a Very Hard encounter, for your group of 5 players (7th level PC's) that have played their PCs from 1st to 7th and are familiar with their characters abilities.
You: As you walk out of the forest, the (CR10 monster) attacks. It's 60' away, 40' up and swooping down from the air. Roll initiative.
(Dice rolled, order sorted out).
You: OK (looks at order) Bob (Wizard), it's your turn.
Bob: I cast fireball at the monster (save made, damage rolled, result noted by you).
You: Ok next is Steve (Rogue).
Steve: I take the Aim action (for advantage) as a bonus action, and then I shoot the (CR 10 monster) with my shortbow. Rolls d20 with advantage, hits. Rolls damage, including sneak attack. Damage noted.
You: OK, it;s the monsters turn. It uses its breath weapon, breathing fire on the entire party (rolls dice, saves are made, damage recorded by the players).
Etc.
The above likely goes on for 3 rounds or so, and then it's done. Really struggling to see how it could drag on past 15 minutes in any sort of regularity unless your players need rules explained to them, are not paying attention, or are spending like 5 minutes deciding what to do, or adding up numbers.
Genuine question, how does that take longer than 15 minutes?