This is fixed since 9269536 in grml-live. The commit message reads:

commit 9269536ba7bb7def8664972d153b71d193450595
Author: Christian Hofstaedtler <ch@grml.org>
Date:   Wed Jun 15 20:27:00 2011 +0200

    Enable --force-unsafe-io for dpkg
    
    While this might help on resource constrained systems, no performance
    impact has been measured during an unscientific test on a well-geared
    machine.

Inspired by http://permalink.gmane.org/gmane.linux.debian.devel.live/11283 to 
improve performance ->

| cat > chroot/etc/dpkg/dpkg.cfg.d/live-build << EOF
| force-unsafe-io
| EOF

We should check whether we can integrate it directly within FAI or otherwise 
integrate it in grml-live and benchmark the difference with and w/o unsafe-io.