If you run WordPress on an IIS Windows server with FastGGI and WordPress is slow, you will need a cache to speed it up and might run into some problems, if so : check out
WP-Cache up and running on IIS on fanrastic.com. A very simple explanation on how to get the Wp-Cache plugin for WordPress running on an IIS windows host, and it works.
Why use a cache ?
Most people skip sites with long load-times, speeding up load times with a cache is a ‘must-have’ if you plan to develop and optimize a site aimed at getting your traffic through a search engine
I installed Google Analytics a week ago and that affirms it, the rate of people that skipped this site after one page was 85% compared to 15% on another domain with normal load times. Pages took 4 seconds or more to load without a cache, and 1.5 seconds or less with a cache. Average page per visitor was 1.14 for this site and 5.8 for the other.
For advertising purposes, long loading time means a site is unattractive to advertisers. You don’t bind visitors and the ‘bad rep’ your site gets also reflects on advertisers, especially with banner ads that work more visual/emotional (image and experience are linked and the poor performance of the site reflects on the advertisers).
Webslug keep a list on slow loading sites, some of them really extreme with 5 minutes load times. Some sites use a five second boundary, sites with higher loading times are considered unacceptable for advertising programs.
A cache also decreases server-load (which isn’t the foremost reason for wordpress bloggers to install it) and allows for a higher concurrent traffic volume, especially if you become popular with a few dozen visitors at the same time, your server overloads.
Loads of reasons to install a cache.
One other reason is MySql, which can handle 2500 concurrent connections but only about 50 concurrent users, problems arise when you for instance have a large file table or registered user table, that for indexing builds a temporary index table (the ‘digg’-effect where a sudden massive rush on the site makes mysql opt out).
Using any caching mechanism diminishes the load on the mysql server which effectively solves most problems with the site slowing down. For wordpress, removing unnecessary plug-ins helps, as does a page-cache.But neither solves the actual problem, the mysql database interaction.
WordPress-MU might in time deliver the goods, a stable mysql query caching mechanism, and for now it seems the ‘ease-of-use’ and the simplicity of the source-code dictate a page-cacher will have to do for the standalone version.
So depending on your plans with a website, where WordPress is the ideal tool for a site with up to, say, 1000 pages, if you plan on a larger business strength site use a CMS that does have backend mysql query-caching.