To expedite site updating, Dreamweaver can create a site cache file to store information about all the links to other files and assets within a site.

The site cache file is created when you select the Cache option in the Site Definition dialog box, and the site cache updates invisibly as you use Dreamweaver to add, change, or delete links to files and assets on your local site.

This [Macromedia] TechNote describes how Dreamweaver MX performs with the site cache enabled when opening a site, closing a site or switching from one site to another. It also briefly describes some of Dreamweaver's cache-related features.

How does Dreamweaver MX use the site cache? (When defining a site)

When the site cache option is enabled, Dreamweaver will build the cache file in memory and save it on the hard drive. The time to create the site cache varies based on the size of the site, so enabling the site cache option could be a performance hit for large sites. When the option is disabled, the site cache is not used.

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Macromedia Technote (16794)