Some relevant thoughts from the blog of Dave Chenault:
He mentions spandex/lycra holding onto water and taking longer to dry, which I think can even be felt when sweating a lot and producing a lot of water vapor on very hot days.
“For 3 season backpacking, Skurka correctly identifies that pants are more often used for leg protection than for warmth. I could see making due with one pair for everything, though two would be better. A ~200 grams/meter pair with a bit of stretch (less than 10% lycra content, such as the Black Diamond Modernist Rock jeans) are good for colder weather and abusive applications, while warm weather pants are ideally in the 120 grams/meter range. Fabrics this thin are not inherently strong, so they should be built as tough as weight requirements allows: 100% nylon plain weave or taslan. Pants like these are not easy to find, fishing pants seem to be the most likely candidates. Aside from gloves and socks, pants are the garment which wears out first and most often. Light nylon pants won’t last forever, perhaps 2-3 years for me, but their ability to dry fast and not cause swamp ass when it’s 85F out more than justifies the cost.”
The RPS pants should work better for very hot weather than pants made with higher spandex/lycra fabric. The Patagonia Tribune Pants have 12% spandex and I’ve even found their 3-7% spandex pants hot, but I guess it depends on the weave and whether there are any ventilation holes or mesh (like in the RR pants).
Like the way higher spandex pants move with you, but not for all conditions. In the same hot conditions my very stretchy Tribune pants are hotter than my Patagonia Continental travel pants, both about the same khaki color. The Continental pants are made of the same half-half nylon/poly blend without spandex as the RPS Pants, and this fabric still offers a little stretch.
I feel the difference in +85F low-wind and especially humid conditions. Also an old pair of Sportif ripstop nylon pants without spandex are cooler in hot humid conditions. So you may find yourself more comfortable with less spandex/lyrca in hot humid conditions.