[jcelerier]

Some tests on my system: including everything at the root of /usr/include: (intel 4770HQ)

    $ for file in /usr/include/*.{h,hpp} ; do echo $file | awk '{ print "#include \"" $1 "\"" }' >> /tmp/test.cpp ; done
    $ (laborious step consisting in removing headers that don't play nice with others... in the end there are still more than 1425 base headers, which end up including most of Qt4 & Qt5, boost, etc)
    $ time g++ -E /tmp/test.cpp -fPIC -std=c++1z -I/usr/include/glib-2.0 -I/usr/include/qt -I/usr/include/glibmm-2.4 -I/usr/include/glib-2.0/glib/  -I/usr/lib/glib-2.0/include -I/usr/include/gdkmm-2.4 -I/usr/include/gdkmm-3.0 -I/usr/include/gtk-3.0 -I/usr/include/pango-1.0 -I/usr/include/cairo -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/atk-1.0 -I/usr/include/qt4/ -I/usr/include/qt/QtCore -I/tmp -I/usr/include/qt/QtWidgets -I/usr/include/qt/QtGui -I/usr/include/KDE -D_FILE_OFFSET_BITS=64 -DPACKAGE=1 -DPACKAGE_VERSION=1 -I /usr/include/raptor2 -w -I/usr/include/rasqal -I/usr/include/lirc/include -Wno-fatal-errors -I/usr/include/lirc -I/usr/include/gegl-0.3 -I/usr/include/libavcodec -I/usr/include/freetype2 -DPCRE2_CODE_UNIT_WIDTH=8 -I/usr/include/python3.6m -I/usr/include/libusb-1.0 > out.txt

    1,82s user 0,18s system 99% cpu 2,013 total

out.txt (the whole preprocessed output) is 710kloc and 23 megabytes.

With clang:

    0,72s user 0,08s system 91% cpu 0,875 total 

so I'd say that in general, preprocessing time is fairly negligible. A few template instantiations will take much more time to compile.