Yes, because focusing on a single throwaway example is productive?
Yup, if you're throwing single monsters at parties that are hyper focused on damage and over powered with magic items, sure, they're dropping the monsters that quickly.
But, the idea that the party is routinely ending EVERY SINGLE FIGHT in one or two rounds? And most of them in a single round? And you don't think that's just the tiniest bit unusual?
Lessee, we're talking 18th level PC's here. So, let's stick with 2014 rules, just for S&G's. 2 Ice Devils (CR 14) are a hard encounter. Note, by 2024 standards, this is a Low difficulty encounter. Right off the bat, since this is a 2014 adventure, we're seeing a big difference. But, let's carry on.
Note, I'm using stock 2014 ice devils here. Nothing fancy. And it happened to be a CR 14 creature I picked at random, and since you complained about using 3 beholders, is there a problem with a group of ice devils? No? Then lets move on.
Lessee, each has an AC of 18 and blindsight out to 60 feet as well as darkvision out to 120 feet. So, right off the bat, there's no way for the party to sneak up on them (virtually) and they absolutely should be attacking from outside the range of the PC's. Dropping walls of ice to divide up the party and break up sight lines. Sure, the party is taking half damage from the cold, but, I should be easily able to make this combat last 3-4 rounds without "kiting" or any shenanigans. Oh, and let's not forget that my Ice Devils can Gate, potentially. And, with 180 HP each, it's going to take 2 or 3 PC's ganging up on 1 to kill it in a single round.
And that's a pretty simple, stock monster in an easy encounter. Again, unless the encounters are all starting 20 feet away, in bright light, I'm baffled how you can think that it's typical that EVERY encounter lasts 1-2 rounds with most lasting only 1.
Finally, let's not forget, that I'm not only pointing at the very strange combat results. There's also the massively over powered magic item list, PLUS resting after each encounter, PLUS using 2014 encounter budgets. And, they are also getting this result from every single module? You think it's far more likely that the system, where no one else is getting these results, is the problem, but, it couldn't possibly be the DM?