Apple's just-released Broadband Tuner allows you "to take full advantage of very high speed Internet connections that have a high latency (5 Mbps or greater). The installer tweaks some system parameters."
According the the Read Me file the program increases the default values for the size of the TCP send and receive buffers. The theory is that larger buffers allow more data can be in transit at the same time. A startup configuration file is added so that the new values stay intact even after restarts.
The system parameters are sysctl variables that are set as follows:
This change has a system wide effect and is applied even if the network is not high speed connection with a high latency, with the exception of modem connections for which the system uses small default TCP buffer sizes.
If you want to tweak you settings by hand, edit the file /etc/sysctl.conf:
I tried it on my Verizon DSL connection at home and it seems to make a difference, but obviously, your mileage may vary.