In addition to improving performance at the HTTP level, here’s a tweak that works for all network activity, not just browsing, at the socket level:
On OS X, open up Terminal (Applications/Utilities), and run the following commands:
sudo sysctl -w kern.ipc.maxsockbuf=8000000
sudo sysctl -w net.inet.tcp.sendspace=4000000
sudo sysctl -w net.inet.tcp.recvspace=4000000
sudo sysctl -w net.inet.tcp.mssdflt=1440
sudo will ask for your login password.