Families vs. project overhead

Although it would be fair to say that two-dimensional components are less demanding on Revit, three-dimensional components are not something you should avoid simply to save on project overheads.

Specifically relating to families, I've found excessive parameters (particularly when nested, with many linked relationships between them) to be significantly more demanding than anything based on object geometry. I'm a big fan of keeping objects generic (at least at the front end of a project), knowing they can be swapped for more specific models when the time comes.

Sure - if an object isn't required in three dimensions, I'd pose the question "Do we need to model it in three dimensions?". If you choose not to, you could use symbolic linework instead (where all but three-dimensional views will show the component). These should be created within the appropriate family template (do not make a habit of using the detail component template where it's not appropriate). In this way the object can still be scheduled from the start. If required, it can still be 'upgraded' to three dimensions later.

In summary, there are many other things that contribute to project overhead - typically not family geometry. Examples are hardware capability of client, server and network, excessive or unnecessary constraints in the model, and in-place components (as opposed to external components).