Common beliefs are that as long as there are little bricks inside of a vehicle that vehicle should not lag, generally this is true, but most lag actually comes from a ratio of volume to brick count, an example would be the laggyness of scaled brick tanks that have a lot of cannon stacking but only about 700 bricks. or how you can actually run a gigantic craft that's more than 4000 bricks when the details and stacking are spread out. So when making crafts brick count isn't everything.

However one place where it is important is collisions, whenever something hits, it seems collisions needs a calculation of the entire craft, more research needed.

Server is lag and lag from multiple crafts is compounded on top of this, but I have observed that server lag comes more from bad host computers more than anything else, (good host computers drastically change performance)

So in reality most bigger crafts lag less than smaller crafts that have details all stacked in a tiny area, identify who is actually causing the lag before kicking!

Bricks obviously still impact game performance, individualistic bricks have calculations of aerodynamics constantly, integrity, weight etc and larger scaled bricks, such as 30x30x10 or something crazy like that would take more calculating power than a normal brick.

What I do for larger crafts is splitting up the larger scaled bricks into smaller ones, poles and plates, it actually improves durability and generates a better look usually.