fast-csg
is still a work in progress.
For large models it works best with CGAL/cgal#5461 in CGAL and used in OpenSCAD by openscad/openscad#4107.
If you want a sneak peek of the new world, enable fast-csg-remesh
in the following build:
( git clone https://github.com/sloriot/cgal \
-b PMP-decimation --depth=1 \
cgal-sloriot-PMP-decimation && \