Unfortunately, this is a limitation of the scroll event on iOS. It does not given intermediate results - like it's debounced - only firing the scroll event when scrolling has stopped. This is a well-documented shortcoming of iOS. As Apple puts it [0], "One-finger panning doesn’t generate any events until the user stops panning—an onscroll event is generated when the page stops moving". There is no workaround at the moment.

[0] https://developer.apple.com/library/safari/documentation/app...