In my opinion, this makes any conclusion dubious. If you really care about compile times in C++, step 0 is to make sure you have an adequate machine (at least quadcore CPU/ lot of RAM/SSD). If the choice is between spending programmer time trying to optimize compile times, versus spending a couple hundred dollars for an SSD, 99% of the time, spending money on an SSD will be the correct solution.
Presumably, 127/128 runs have both the test file and the single header file in memory cache, so the distinction is moot.
Also, I find the conclusion that we should all just buy top end machines and ignore performance problems that don't manifest there fairly unconvincing. I think that kind of thinking is responsible for a good chunk of the reason the web is so bloated today. :-)
You don't need to develop on a $200 notebook to care about performance.
For any kind of even vaguely profitable software, your developers should all have kick arse machines.
But they should test on a $200 laptop :)
I've seen this problem a few times, someone looks at their N core machine with M GB and says, oh look i'm only using 3/4 of M so when I buy the 4xN cores machine I'm going to put M ram in it again. Then everything runs poorly because the disks are getting hammered now that there are another 32 jobs (or whatever) each consuming a GB. Keep adding ram until their is still free RAM during the build. Its going to run faster from ram that waiting for a super speedy disk to read the next c/.o/etc file.
Note that Visual Studio, for example, does a poor job of this because it only spawns one compilation per CPU thread. This results in individual threads being idle more than they ought to be.
I've just tested one of my ~300 KLOC C++ projects, broken into 479 .cpp files and 583 .h files.
Using Linux (GCC) after dropping the disk cache, on a 5400 RPM HD, the full build on 14 threads took: 78 seconds.
On a fast SSD (same machine, after dropping caches again) it took 61 seconds.
Linking was ~7 seconds faster on the SSD, so arguably you could say that actual compilation wasn't the same ratio as fast, but overall build time is most definitely faster.
Source was on the same drive as the build target directory.
At a previous company I worked at, we got SSDs to speed up compilation (and it did).
If it doesn't make a difference, all that means is that your project is small, or doesn't have too many dependencies. Good for you. But that's not the reality for all projects.