Slightly topic-adjacent, but this just occurred to me somewhere else, and I thought it might be relevant.
One of the issues with combats that drag on too long is that, if a fight is interesting and balanced when everyone is active, then it becomes unbalanced as soon as someone drops. In order to prevent that tipping point from happening in the first round, monsters are designed to withstand multiple rounds of focused fire before dropping; but since they have so many HP, it takes just as long to go through each one of them as it does to go through the first one, except the exciting part of the fight is already over.
One way to prevent tedious combat is to ensure that enemies die in quick succession, by preventing focus fire. If the whole party isn't attacking the same enemy at the same time, then the enemies don't need enough HP to withstand that assault for multiple rounds, and all of the enemies should die in rather quick succession since you're whittling them all down at a constant(ish) rate. At least in theory, that means the whole fight should stay interesting, because the fight is over shortly after the enemies lose action advantage.
The hard part is figuring out how to encourage spreading out, when focusing fire is so obviously the way to win. I figure you could probably do it by assigning penalties to every attack in the round, past the first, against the same target. It would probably take a bit of testing in order to get it right, though.