Lag with 100 parts

[Padishar]

The resource handling inefficiency applies to all parts that produce or consume resources. If you build two similar craft but replace the fuel cells with rtgs in one case and passive part in another then you will see a noticeable performance hit for the rtg case. With large numbers of parts in the vessel the issue gets worse.

I'm not saying that the physics calculations don't take a considerable amount of cpu (they do) but there are other areas of the code that can take considerably more in some cases.