Where's the second part of the story: where someone else profiled and discovered 19 of the linker scripts were generated really fast, and that re-working generation of the slowest script only took 15 minutes.
I have been profiling and it’s really just python being slow.The first ⅓ of the run of 12 cores is all python. Fortunately these are mostly independent so there is currently no blocking, but there eventually will be.