Using gzip compression can decrease the number of bytes sent by the server. This gives the perception of faster pages and also cuts down on bandwidth usage. Depending on the data sent, how well it can be compressed, and whether the client browsers support it (IIS will only send gzip compressed content to clients that support gzip compression, such as Internet Explorer 6.0 and Firefox), your server can serve more requests per second. In fact, just about any time you can decrease the amount of data returned, you will increase requests per second.
To make this change, you are going to need to edit the Metabase. Edit \Windows\System32\inetsrv\MetaBase.xml with your favorite text editor. Search for "IIsCompressionScheme." There will be two XML elements, one for deflate and one for gzip. Both elements have properties called HcFileExtensions and HcScriptFileExtensions. These contain a space-delimited list of file extension for compressible content. At a bare minimum, you will need to add aspx to the HcScriptFileExtensions list. Note that if the properties are left blank, then all content, regardless of file extension, will be compressed.