Builds on MacOS 10.11 have been painfully slow (for instance 23 minutes to build afw instead of the more typical 12 minutes, 30 minutes to rebuild ip_diffim, 20 minutes to rebuild meas_astrom) and much of this time is spent running fc-list in 8 cores.
I suspect matplotlib is triggering this process, but I have not verified it.
I see this using lsstsw to build a fresh stack and with manual builds.
A workaround is to repeatedly kill fc-list, e.g. with this bash script:
I checked my fonts with Apple's Font Book and found a few dozen with "minor errors" that I deleted, but nothing serious. I'm still seeing the problem.