Mavericks agency

Greetings from Ukraine! I am trying to recreate the parallax section effect (https://ochi.design - from here. right after the preloader when u start to scroll the page), when the bottom of the section reach the bottom of the screen the next section should scroll up a bit faster than previous and to overlap thereby the previous one. I achieved what I wanted (on my website it works smoother then in codepen tho) but I'm not sure it's the best way to implement this. I'm curious how can I improve my code and optimize it. Thanks! WATCH IN FULLSCREEN MODE