Weighted Scroll Effect

Good morning to you all

I’m not well versed in coding by any means but I’ve been searching for an effect to emulate the scrolling function that can be found on SLAPS . I noticed that when you scroll on the site there seems to be a weight or delay that makes the scroll feel heavier, I hope I’m describing it well enough for somebody to understand.

My only guess is that this is a custom coded feature, if anybody could point me in the right direction I’d be more than grateful.