Just to chime in here... Yosemite on my Mum's 2015 (2.9GHZ I5) 13 inch Retina Pro and brother's 2014 Retina Macbook Pro is laggy and jerky at times... I don't get that sort of thing on my Mid 2012 Classic Macbook Pro, however apparently El Capitan fixes most of the issues. Haven't yet installed it on either of their machines yet, waiting for 10.11.1 or 10.11.2 just to be on the safe side.
I've used a lot of Retina Macbook Pros throughout the years, be it performing maintiance/upgrades on client's computers or for friends/family or fiddling around at the Apple store and I've always, always, always been annoyed by the lag/jitter that not everyone perceived. Put simply, I've probably used 30 or so Retina MacBooks (mainly 13 inches), 2012/2013/2014/2015 models and I've seen it on every single one, if I use it for longer than about 15 minutes.
I find the same thing with the iPad Air 1 and iPad Mini 2. I've used heaps of them and under iOS 8, jittering has annoyed me on every single one of them, including the Apple store ones.
I'm very inclined to think that some don't notice the jitter/lag and some do.