The simplest answer is that it’s the total number of pieces/items/objects that make up your base.
Client server communications are basically the same whether it’s a 1,000 foundations, 1,000 pillars, 1,000 ceiling tiles, whatever combination of pieces you have.
The same thing is true for your graphics card. Having to render in a simple square box with 1,000 pieces or a complex design with 1,000 pieces is still 1,000 pieces.
It’s important to remember that “rendering in” will include many items that you can’t see. Everything within render range of your character gets communicated from the server to your client, everything within render range gets calculated by your graphics card even if you can’t see it.
I’m sure that, if you dig down deep enough into the technical details, it’s possible to calculate or figure out which objects are possibly slower, or whether certain building shapes are slower, but honestly that’s not worth your time or anyone else’s. Computers do things in milliseconds, whereas your and I process in whole-seconds. While there is technically a difference between 1,000 foundations and 1,000 pillars, the difference is so small that it really doesn’t matter to you.
If you just keep in mind that “more pieces = slower” that will help you when you build.
And, of course, this also means that fancy bases with lots of decorative items are slower. If you build a 5x5 base and pack it full of 1,000 decorative vases and paintings it’s going to take roughly the same amount of time to load as a base with 1,000 foundations+walls+ceilings.