All the vaults and other interiors are made up of multiple cells, I assume this is for better performance but there are other ways for it. Or does it mostly affect the loading time of the cell? But I am working on a completely new vault now, existing of: Entrance, weapon room, clothing room, living quarter, a bar. And possibly more, should I create every part in a different cell? Or can I use other methods to optimize it?

Bigger cells take more time to load. Especially if you have a lot of unrepeated objects. Look at the size of vanilla cells to get a good idea.

I have a vault that has around 15 different cells. But then, its a very large vault with a lot of unique objects and retextures.

And, there is a way to optimize a cell so only certain cells render. I cant remember what its called at the moment though, I had a 12 hour day and work and I am exhausted. (Room bounds or something like that.)