On my Debian Wheezy system I noticed very slow HTTP connections, for example while using apt-get or wget. The problem was somehow related with the uncomplicated firewall (ufw), since it only occured while the firewall was running. After searching forums and mailinglists, I found the advice to turn off IPv6 support, which turned out to be the solution (or at least a workaround).
The Debian Wiki shows how to disable IPv6 support in /etc/sysctl.conf by adding the follwing lines:
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1
net.ipv6.conf.<YOUR NETWORK INTERFACE>.disable_ipv6 = 1
Find out which network interfaces you have by using ifconfig.
Of course, a better solution would be a working ufw with IPv6, but for now IPv6 is not important for me and I hope that ufw will introduce proper support for it soon.