I just set up some simple vertex/fragment shaders for handling lighting instead of using the openGL calls. I have a few terrain VBOs setup that are drawing a million or so triangles for testing (bunch of random spikey ground). I noticed that if make the spikey ground spikes higher by multiplying all of their heights by 5, I see the FPS drop from about 98hz to about 44hz.
Searching the web, I found https://www.khronos.org/opengl/wiki/Performance which says "Fragment processing takes longer for large polygons than for small ones", which would explain why I see the decrease.
However, I can't find anything that says why. So my question is, why does fragment processing time vary based on the size of the polygons?