I know it seems complicated. Here's a rough rule of thumb on my computer which is very slow by today's standards.

If you have 1 million pixels to shade, 20 nodes will add 1 second.

Of course even if you render large, you rarely fill it with skin. So even a 1600 x 1200 portrait usually has no more than a million skin pixels in it. Let's say we add 5 of these math nodes - it would take about an extra 1/4 second to render versus using the skin texture alone.

These math nodes are not slow at all. You barely notice them. However, some of the "natural pattern" generating nodes (called noise nodes) are somewhat more expensive. They run about a second each per million pixels. Adding 3 or 4 to make, for example, freckles or something will increase render time noticeably. Then of course there are the Ambient Occlusion nodes and other ray-traced effects. But you can't get something for nothing.

My most complicated shader has about 140 nodes in it when every feature is turned on (freckles, moles, veins, pores, etc.). What would otherwise be a 40 second render increases to about 80 seconds. If you turn on raytracing for AO it jumps up to 2 minutes, but that's not the math nodes doing coloring. That's just the extra work of a high quality image.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)