There are a variety of reasons why web browsing can be slow and as such, if this happens you can do several things to help combat it:
1. Try OpenDNS servers
The DNS servers for your connection are provided by your ISP, and their performance may vary depending on how the servers are configured, how much traffic they are getting, and what other processes are running on the same systems. Adding OpenDNS servers (188.8.131.52, and 184.108.40.206) to your system should speed up the process by which domain names are translated into IP addresses, and allow your system to load pages and web resources without delays.
2. Reset Safari
Many times corrupt application resources are the reason for an application's slow performance. Safari has a variety of these, and Apple has conveniently enabled quick resetting of these resources through the "Reset Safari..." option in the "Safari" menu. Despite this, some people may wish to still reset Safari's resources manually. To do this, locate and remove the files in the following folders:
Then remove the following from the /username/Library/Safari/ folder:
3. Delete caches
Safari enables you to remove its cache from the "Reset Safari" option; however, you can use a third-party cache cleaning utility built for your operating system to remove all system caches to clear any faults with them that might be affecting Safari. Several of these utilities are: OnyX (currently 10.5 only), Leopard/Snow Leopard Cache Cleaner, and MainMenu.
4. Remove plugins.
If a plugin is not functioning properly it may cause Safari to run slowly, especially when loading PDFs, images, movies, and other media files. Try removing all third-party plugins from the /Macintosh HD/Library/Internet Plugins/ folder, and /username/Library/Internet Plugins/ folder. After this, relaunch Safari to see if the problems persist. Then install the latest version of the plugins you use.
On a slightly different note, I've found when browsing Apple's support sites and discussion forums that Safari will sometimes not communicate properly with the Apache web server that is hosting the support sites. When this happens I get one of these two errors:
This problem seems to be with how Safari is handling cookies for the website, and deleting cookies reduces the prevalence of these errors; however, they begin to occur again after a while. Sometimes the only way to clear the problem is to quit and relaunch Safari. This appears to be a problem that is specific to Safari, since it does not happen in other browsers, and hopefully will be addressed by Apple soon. However, in the mean time if a fix or workaround surfaces, I will post it here.