Oh good. This is my area of expertise.
OK, so this is going to be a little bit complicated - I am going to attempt to simplify it. In Fallout 4 there is a set list of objects that can be scrapped. Anything not in this list is unscrappable. To make the game run at an acceptable speed, all of these are combined together into two data objects called previs (what can we see?) and precombines (what's here that we can't scrap). These two systems are essential to make the game perform well. Precombines allows the game to draw lots of objects together. This is fast. Scrap mods (excluding Raze my settlement / Clean my settlement*) disable both of these systems in settlements. They also expand the list of objects that you can scrap. The downside is now that the game must draw each and every object, in a 5 cell by 5 cell grid around you. There are many, many objects that can go into what looks like a simple model. Drawing them one by one makes things very slow.