Thats not how it works. Having more shapes doesn't inherently cause lag, its what the shapes represent. Think of it as # of faces rather then the shape itself.
If you build in a manner to avoid wasted faces building the same thing with less "parts" but the same amount of faces would result it no change.
The relatively small amount of wasted faces on an average model far outweighs the negatives of building with more complex geometry and time would be better spent replacing physical detail with textured details.