cygwin project provides an efficient and POSIX-compliant "user-mode"
implementation of posix_spawn [1] for Cygwin.  Using this facility
improves process-creation performance considerably, especially for
large processes:

    ~/cygspawn
    $ sh bench.sh
    + env ITER=1000 ./testspawn true

    real    0m4.508s
    user    0m1.275s
    sys     0m2.675s
    + env ITER=1000 ./testfork true

    real    0m8.845s
    user    0m2.847s
    sys     0m5.230s
    + env ITER=1000 JUNKBYTES=209715200 ./testspawn true

    real    0m5.179s
    user    0m1.550s
    sys     0m2.661s
    + env ITER=1000 JUNKBYTES=209715200 ./testfork true

    real    1m57.523s
    user    0m3.049s
    sys     1m52.339s

All posix_spawn functionality is supported, and no special
configuration is required.  This library must be dynamically linked
into any program using POSIX spawn.

To install:

    make
    make install prefix=/usr

To build programs with posix_spawn:

    gcc -o foo foo.c -lcygspawn

[1] http://pubs.opengroup.org/onlinepubs/009695399/functions/posix_spawn.html