@Fabito48hd The difference is;
Assuming the author also know the difference and write the mod's name correctly,MLO contains it's own 3D models and collisions. If let's say there is a MLO house mod, the entire house structure being a single 3D object. Some seperates the interior and exterior as 2 different 3D objects, some divides some stuff, like the shell of the house, interior, exterior, props (like couches, beds..). BigShaqNOKetchup's MLO mods are great example for this and they all have a very good performance.
Because the MLO mod being a single (or a few) meidum-big 3D object, it's having a very significant higher fps compared to ymap. There is a mod for Fallout 4 that is combining all the stuff in an area into 1 big 3D object instead of having them seperately and that mod gives over 30-40 fps boost which is amazing.YMAP is putting lots lots of multiple small only vanilla (90% of the time) or both vanilla and add on 3D objects together piece by piece, eventually when all put together they are making a small-medium-big object like house. Now some mods have some 3D addon objects in themself, however this is not to be confused with a real MLO. Those objects are being small sized things just as a complementary and they are not having any better performance than vanilla 3D objects.
For this reason while a YMAP mod eating a lot of fps (depending on how many objects placed in it) while a MLO mod taking a very slight fps. The simple logic is "less objects=more fps". Master Yoda says "Size doesn't matter." and he is a very very wise Jedi
But even MLO has it's limits at some point. This is why massive sized addon MLO maps eating too much fps. Since the entire map being 1 massive sized object, the LOD distance can't be set properly. You have to set the highest quality LOD distance at least from the center to the farthest corner of the map so it doesn't show low quality LODs at least while you are on the map. and such a far distanced highest quality lod size of a massive sized 3D object can eat a lot of fps. This is why you should always place the maps far away from the game's own map so at least the game engine only loads the lowest quality LODs of the vanilla map for maximum performance. But despite this thingy, as long as it's not a very simple map with little amount of objects, MLO maps still offer higher fps compared to YMAP maps.