It should be slower, and testing it as an override material it was. 46 seconds for vanilla Diffuse only and 52 seconds for vanilla Principled - I did expect it to be more. Principled also does glossy by default, so it makes sense to me that it is slower. Diffuse only tends to be brighter overall, and especially in corners where diffuse provide a uniform spread of the light bounce. Enabling caustics to bring back the lost energy from glossy bounces, it becomes as bright as diffuse only, possibly even brighter - this is probably a result of Principled not conserving energy correctly.
At this point, I’d say it’s an artistic choice. At specular roughness 0.5 and above, it hardly matters visually if you show diffuse only or “fresnel is everywhere”. Which is technically true, but all the other stuff we tend to ignore has to be there as well for it to really matter. The only point where you don’t want to use Principled is where you want the Oren-Nayar characteristic of Diffuse shader (Diffuse roughness > 0). Or in some cases where you use Velvet shader, because it has it’s own very special kind of “specular”.
Diffuse vs Principled - doesn’t matter that much in my view. However, learning to use Diffuse is of great importance, because you may want to mix it in with shader types Principled doesn’t have, like Translucency. And fresnel mixing manually you need to make sure things looks correct (or correct for it) from both sides as fresnel is sensitive to normal (invert IOR for backfacing faces). Also note that fresnel doesn’t lower it’s output based on roughness. You need to build yourself a PBR version to do that. And even that won’t have the same response as the Principled one (at least not mine).