With $CFG->pathtosassc commented:

$ sudo port deactivate php73-xdebug @2.7.2_0+debug

$ time php admin/cli/build_theme_css.php
== Build theme css ==

real	0m42.924s
user	0m41.264s
sys	0m0.575s

$ sudo port activate php73-xdebug +debug

$ time php admin/cli/build_theme_css.php
== Build theme css ==

real	2m16.842s
user	2m12.777s
sys	0m1.015s