And finally, one more thing that bothers me most. In the very first scenario I described in this thread, with the plane mesh filling the whole screen and taking 190ns to render, I noticed that depending on how close the camera was to the plane, the render time varied, despite the fact that the plane was the only geometry entity filling the whole screen. I noticed that the closer the camera was to the plane, the less time it took to render it. My times varied from 180ns (as close to the plane as possible without frustum near plane clipping) to 220 ns (as far as possible to still fit the entire plane on the screen).
Why is that a surprise? You sound like you somehow moved from thinking that only the number of polygons matters to thinking that only the number of pixels matters. You get closer to the plane, more geometry gets clipped, fewer triangles get rasterized -> less overhead. You are measuring ns. There are about a gazillion factors that come into play, many of them hidden from you by things the driver might be doing or just the way stuff happens in the hardware.
How are you even measuring the exact timing for vertex and pixel processing when these things should be processed in a pipeline on the hardware? If you have more than one triangle, it's not first going to process all vertices and then do all the pixels. One triangle is done and gets rasterized/shaded while at the same time the next triangle gets prepared. There are caches, the second time around a vertex might not even be processed anymore. Hardware today has unified shaders that can be dynamically used for either vertex or pixel processing, completely changing how long vertex and pixel processing will take.
There is little to no point to make ns measurements and expect any obvious and linear relationship between numbers. Not on modern hardware.