TileEntities are not necessarily rendered with TileEntitySpecialRenderers. The machines in RotaryCraft are complex and animated so they are rendered every frame by a TESR. The modular pots and plant "microblocks" in my Garden Stuff mods, on the other hand, render everything statically with ISimpleBlockRenderHandlers. Those only get run when the chunk needs to re-render (which by the way, also renders stuff you can't see -- notice how you could always see caves when a chunk failed to load?)
If you're using the microblocks from ForgeMultipart, it looks like they do both. They have a static and dynamic render part. The code is in Scala and really hard for me to follow, so I can't say much more beyond that. I think the multiblocks themselves are rendered statically and the dynamic portion is to interop with dynamically-rendered blocks. But even if nothing dynamic is rendered, there's going to be some overhead processing involved to check that and skip it.
However TileEntities are rendered, one thing that's universal is they carry a lot more data overhead than a normal block, which must be saved, loaded, and pushed over a network. Spam enough of them and you will feel it, even if it's not in the render code.