Javascript Parallax Scrolling Text
According to my experience, web animations don't use CPU as such, but rather prefer the GPU at times for rendering. On phones, custom animations like yours will run without any moderation of fps, and so a lag is kinda expected. On top of it, js doesn't use parallel threads of course, so it won't even use the remaining core if they are available. 

What I'd suggest is use some good library to run these. I remember using Three.js and it ran on my Lumia 520 at 8FPS but still without lag, as the animation was optimized for low FPS. The same ran on my PC at 60FPS smoothly just the same. 

Hope this helps.