Ok, so I recorded video in slow-motion to show how it looks - it's hard to record default video as 30 fps not enough to show difference.
Best you can see on 8s when I'm swiping up -> there is big jump in text display and 2m 19s when it goes smoothly.
After scrolling for few seconds down& up at some point scroll started to work like there is 20fps - text jumping - you can see until 1.40 m that text when scroling jumping like 6px to next position, after I opened bottom bar( IOS control panel) on iphone - scroll got back to 60fps. In slow motion it looks like text jumping for 1px not 6px. This fps drop happens only when you touching with finger screen and making up or down slides, when u releasing finger scroll looks fine until u again touch screen.
I seems found best way how to reproduce it, open Cocos2d-x cpp test app(in portrait mode) with that menus in scroll, click any of menus option to change scene and then after 1-3 second click back button to return to menu scroll -> when trying slowly scroll down or up(hold finger on screen all the time) you will see scroll lagging like there is 20fps - if you open and close IOS control panel -> scroll back to normal or randomly itslef goes to normal state- until again clicking menu option-> back and again scroll starting to lag.
It even looks that not scroll lagging itself but touch events are sent with delay and like interrupted all the time -> so text seems jumping thru skipped pixels when making scroll and it makes 20fps feel as it is not smooth.