Ubuntu 10.10 (Maverick) now allows for a btrfs root filesystem. Even before the final relase, performance problems with dpkg have been spotted. The short answer was:

sudo apt-add-repository ppa:brian-rogers/btrfs
sudo apt-get update
sudo apt-get upgrade

And although I've experienced a similar performance impact before, it's somewhat better now, though I'm unsure why: This is still Linux 2.6.35 and dpkg hasn't received any upgrades either since then.

# ls -lgoh ./openoffice.org-core_*.deb
-rw-r--r-- 1 27M Sep 30 08:05./openoffice.org-core_1%3a3.2.1-7ubuntu1_amd64.deb
# echo 3 > /proc/sys/vm/drop_caches
# LC_ALL=C time -p /usr/bin/dpkg -i ./openoffice.org-core_*.deb
real 47.61
user 2.09
sys 7.52

# dpkg -i ./dpkg_1.15.8.4ubuntu3-nosync1_amd64.deb 
# dpkg --force-all -P openoffice.org-core
# echo 3 > /proc/sys/vm/drop_caches 
# LC_ALL=C time -p /usr/bin/dpkg -i ./openoffice.org-core_*.deb
real 31.34
user 2.58
sys 6.50