Do Variant Dungeon bosses have more sustain than Dungeon bosses?

I haven't done the normal Variant Dungeons much, but one thing I've noticed is that while I really enjoy the boss mechanics, the bosses themselves seem to take longer to kill than comparable normal dungeon bosses.

The only boss I've fought enough to feel absolutely confident that this wasn't just a case of "still learning" is the giant midway through the Sil'dihn Subterrane — the one who flips the explosive barrels such that you have to stand away or next to them. Each time I do that fight it seems like everyone in my group is doing the mechanics near-perfectly, but the fight keeps going well past the point where the boss has repeated his entire identical rotation of moves several times, such that by the end I'm just relieved to be able to move on. Which is notable because boss fights are my favorite thing in FF14, and outside of these Variant Dungeons I can't think of any times in recent memory when a boss fight has felt like it outstayed its welcome to anywhere close to this degree.

So I wanted to ask other players. Do Variant Dungeon boss fights take longer than most fights? Am I just getting unlucky with groups? Or is there something else going on? I'm curious to know more.

It sounds like your parties just aren't doing as much damage as they could. Soloing VCs on sage I rarely see each mechanic more than twice, and the bosses almost always die before they can complete their second rotation.