This mite sound like a dumb question but does a completely transparent shader still make a processor work as hard as rendering the other types of shaders?
It will probably make a processor work harder as the processor needs to do extra work to calculate the light paths and how the transparent shader refracts light.
Guessing here, as I don’t know the exact details, but transparent shader should be pass-through, rays continue without any additional processing. There is no refraction in transparent shader and light path type is just a switch based on shader type, normal direction and bounce/intersection count.
It does still force transparent shadows though, which is a pretty big speed hit too. Plus you still have to do the intersection test in the first place. It’s definitely a lot slower than just not rendering the object.
Yeah, there is no escaping from the basic things. But in addition to these, transparent shader should not do much more, so the speed difference with other shader types comes down to the balance between time spent on the core functions and time spent on shader specific stuff.