To optimise the material noodles, I usually try to avoid extra shader nodes—that are slow to calculate—and use instead colour mix nodes.
The difference in speed is evident, but in a recent test I have noticed that the results are slightly different.
Here is my test setup:
In the yellow frame two glossy mixed with fresnel, in the blue frame one glossy with the roghness driven by colour mix + fresnel.
For the roughness value I have used the «value» of the HSV tab of the colour input node, that AFAIK should be linear. The values are shown in the unconnected value nodes.
Here are the results with 144 samples:
two glossy + mix shader 00:51.81
one glossy + colour mix 00:47:20
In the following post another image.