Animations need to to serve a purpose.

Fading in is justifiable when you're adding new content on top of existing content, and need to draw attention to the fact that something changed in that part of the screen. None of that applies in the case of fade-in during scrolling. The user is already scrolling, everything on screen is moving, and new content is already expected to be coming in to view at the bottom of the window. Adding animations on top of all of that doesn't help anything, and just distracts from and delays presentation of the content the user was already trying to reveal.