cowbuilder
This is the tutorial for cowbuilder. See also [:qemubuilder: qemubuilder].
troubleshooting
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 the location /tmp/new is something else. Try those commands and see how fast they are on your computer. You can compare them to our [:cowbuilder_benchmark: benchmarks].