Have you ever actually proven that asynchronous reading of sources actually speed up compilation? 

Over in gdc, I've never seen this to be the case through the various random time reports I've taken over the past 2 years for druntime/phobos library builds.  Both in single/separate compilation of modules (what gdc build process does), and whole compilation of all phobos modules at once (what dmd build process does).  Or if there is a speed up, it is frankly negligible. 

So for me I've always had asyncread turned off. Though the dependency of the thread  library was another factor taken into account.  :-)