There may be times when Internet access seems to crawl, even when you are connected through a broadband connection and do not have other people on your network using bandwidth. While there are several reasons for this, one could be that your ISP-supplied DNS servers may be underperforming.
The use of DNS servers is how your computer resolves names (i.e., www.macfixit.com) to a specific IP address, to which your computer can directly connect. Name resolution is done by an interconnected network of DNS servers that catalog and connect a distributed hierarchical database of domains and subdomains. The crude basics for how this works is (in looking at macfixit as an example) the DNS system first looks into the domain "com" for any server lists assigned to "macfixit," and then looks in that "macfixit" subdomain for any that are assigned to "www." In this process there is a lot of caching, redirecting, and other communication between multiple name servers, and many times ISPs do not have their DNS servers set for maximum performance.
One easy way around this is to use an alternative DNS server that is optimized. These include private DNS servers (non-ISP ones that are set up by companies) and some public DNS servers such as those from OpenDNS and Google. Not only will this help Web access, but will help other Internet-based services that show performance lags as well, such as Mail as is mentioned in this Apple discussion thread.
Changing your DNS servers can be done in the Network system preferences, but keep in mind that it will need to be done for every used logical network interface (those listed that are used for network access). If you use both the Airport and Ethernet ports, and set custom DNS servers for Airport and not for Ethernet, then when you connect via your Ethernet port you will not be using the custom DNS servers. An alternative is to set the DNS servers in the local router or gateway that you use, which will then have them be the default servers used by all members of the local network.
For specific information on optimizing DNS and changing the primary servers your computer uses, see our article "DNS security and performance considerations, and ISP alternatives," which shows how to change the DNS servers and has some suggestions for alternative DNS servers to use.