iPhone 6S Plus. No stuttering or lag whatsoever. You can watch it in 60FPS.
My 6S Plus is definitely not that smooth on landscape mode, not even with Reduce Transparency set to On.
I'm currently using Reduce Transparency ON and Zoomed view and it's blazing fast, but as soon as I go back to REduce Transparency OFF and Standard view, the 3D touch peek on icons goes at like 5 fps. horrible.