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Xara does not let you publish individual pages. So your best bet is to use the setting Egg has posted a screen shot of.

Xara maintains a file on the server which it used to determine what changes have been made and will publish only those changes. What does take some additional time is after the publish, Xara deletes objects that have been replaced or deleted from the site. But publishing changes after the main site publish should be much faster when you have Fast Publish enabled.