As I read it, the flag changes are to find the source of the slowdown. I.e. if disabling LTO for a moment would improve the compilation speed drastically, then

As I read it, the flag changes are to find the source of the slowdown. I.e. if disabling LTO for a moment would improve the compilation speed drastically, then I probably still want it enabled but may now want to see if there are ways to "please" the LTO, to massage the input to be somehow more LTO-friendly and allow it to work faster - or find some other ways to optimize LTO (like how they experiment with alternative linkers in other parts of the article). It helps to understand whether the biggest gains can be had from focusing on optimizing this particular area. Also it helps quickly and roughly establish what's the "best case" improvement they can probably achieve, as presumably the debug-build set of options is bent on compilation speed. Still, in the end the biggest performance gain seems to have been found faaar away from LTO in this case.