cowbuilder is a wrapper for pbuilder that creates environments by hardlinking directories instead of untar'ing them (cp -al <template>/ <chroot> instead of tar xf <template>.tgz). For an introduction to the pbuilder software stack, see pbuilder.
Slow copying and removing of the COW directory
What cowbuilder does is:
cp -al /var/cache/pbuilder/base.cow /tmp/new
rm -rf /tmp/[new]
Of course cowbuilder uses a different location than /tmp/[new]. You need to optimize those 2 commands on your computer. They should take no more than a few milliseconds each.