I rode Conti Duraskins all winter, and they've held up great. But on a whim I recently picked up some simple rubber tires sans-Kevlar (Kenda Koncepts to be precise) on the cheap, and I feel much faster than I did in the Duraskins. Do kevlar-belted tires really slow a body down, or is this a tire placebo effect? Both tires were 700x23, inflated to max pressure. Hollaback, girls.
Yes and no. I've had mixed results with reinforced tires. For example, the Bontrager Race Lite Hard Case tires (with multiple points of reinforcement, a kevlar belt being one of them) are most certainly not fast tires. On the other hand, Conti GP 4000s are made with Vectran, a material 6 times harder than steel and more effective than kevlar. They are quite nearly the most supple foldable tires out there.
There are definately more factors too it. Rolling resistance is one. I can also think of tire pressure, tire width and diameter. But, from first glance I would think that a Kevlar belted tire (not beaded) would weigh a bit more than non Kevlar tire. That is why I suggested to weigh the tires.
I am fortunate. I work in a lab in a food plant. I can accurately weigh anything from a grain of sand to a semi. I weighed my 29er tire yesterday because I am converting to homemade tubeless. The manufactures spec is 570g. My calibrated scale at work says 640g.
So even if you look up the manufactuers spec, they are only a guidepost.