As far as the intensity of this show, there are a few things to be said about it.
First, this is a REALLY long 1-acter. 1:45 with no intermission is a lot, especially when you combine it with, second, nobody leaves the stage very often and it's often only to get ready for their next scene. There's definitely a lot of run off, costume change, gulp of water, and back on moments in this show for the Thoughts. Third I'd bring up that the Thoughts aren't just the ensemble, they also play all of the supporting roles. This is far from the only show where that's the case, but it does add a lot to have multiple roles you're playing every night. Finally, I'd just add that A Strange Loop is a VERY emotionally intense show. It's a show that would be a lot for anyone, but I'm sure it's especially intense for a group of Black Queer people, many of whom are probably reliving really tough and potentially traumatic incidents from their own lives every night. That can take a lot out of you.
That's not to say it's more or less challenging than being a chorus girl in a show like Some Like it Hot, but I don't think it's the same thing. A dancing tour de force like SLIH can be very hard on the body, but it is VERY different than what would be intense about performing A Strange Loop.