It's not that it's unwieldy, it's that cotton absorbs and holds onto water, so if you fall in water on a hike, or get soaked from a downpour, you're going to be stuck in cold, wet, pants.
It is, however, possible to coat the fibres with something to make them not do this, perhaps that's what the 'performance denim' did.
Basically, if you have a chance to be stuck far from indoors with soaked clothes, you want them to be made of a material that doesn't hold water as cotton and wool do, it's fairly easy to find nylon, acrylic or polyester pants.
I think they were thinner and lighter so they would probably absorb less water even if fully soaked. I think they prevented absorbing water with some kind of a coating too, though.
It is, however, possible to coat the fibres with something to make them not do this, perhaps that's what the 'performance denim' did.
Basically, if you have a chance to be stuck far from indoors with soaked clothes, you want them to be made of a material that doesn't hold water as cotton and wool do, it's fairly easy to find nylon, acrylic or polyester pants.