[Also when giving one graph for one type of build, could you also give a similar graph showing how it looks with dash or ksh or whatever you used?]
Graph for the gcc will be exactly the same. Only overall time will be shorter.
ksh93 is not the good replacement candidate. I've tested it with gcc and using it causes that whole build freezes at some point. I've started my testy from ksh and this is why I've took next on the alphabetic list which was mksh.
Maybe it is only some issue with Fedora version and latest ksh93 is fixed .. really I don't care about that as better POSIX sh alternatives are around.