With $CFG->pathtosassc = '/usr/local/bin/sassc';

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

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

real	0m29.985s
user	0m27.802s
sys	0m0.877s

$ sudo port activate php73-xdebug +debug

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

real	1m42.369s
user	1m39.313s
sys	0m1.060s