It is definitely perceptible. Compare a normal iPad (60 Hz) to an iPad Pro (120 Hz), the fluidity of movement is very apparent in just playing around with the home screens.
It may be perceptible, but the number of factors that could affect the different systems in that comparison means it's not exactly a good test. At the simplest level, there's no guarantee it's actually even rendering updated frames at the rates in question if it's limited by some other factor, and the differing hardware may change at what point that limit is hit.
Yes, what I was trying to get at is that just because the hardware is capable of 60 frames in a second, that doesn't mean the software was delivering 60 frames a second. The iPad Pro has a different processor than the iPad (A10X Fusion vs A10 Fusion), and in a lot of tests it's significantly faster.[1]
The iPad pro does have more pixels to push around, but that doesn't exactly negate the CPU difference, it just makes it more complicated to draw an actual comparison. That that's before we even get to the actual graphics processor, which itself could do a better job of offloading some processed to hardware (better OpenGL/Metal/whatever support). For all we know, you were seeing a average of 35 updated frames a second on the iPad, and you're now seeing an average of 55 updated frames on the iPad pro. In that case, the doubling of the screen refresh might help a little (in reducing noticeable laggy frames a bitas it can update between what would be frames at 60Hz), but it wouldn't be earth shattering. I doubt it's that bad, but as an example, this should show how a Hz rating on what a screen is capable of doesn't mean much.
The real benefit of higher screen refresh rates is to better support different lower native refresh rates. Much video content is at 24 FPS. A 30Hz or 60Hz screen can't represent that faithfully, and will need to double some frames. A 120Hz screen can perfectly represent 24 FPS content[2], and that's the real reason screens (and TVs) ship with that refresh rate. Different media (television, internet video, DVDs, Blu-Rays, video game systems, etc) all have different refresh rates they want to deliver.
