There is an issue on sites with over say 500 nodes, that the sitemap can take a long time to produce (around 8 seconds) which means some search engines consider the sitemap page as a timeout, which kinda defeats the point, right?

This has been discussed in the following thread - http://our.umbraco.org/forum/developers/xslt/21607-Slow-XSLT-for-Sitemap?p=0#comment83905 and a fix has been proposed (I'd quite like to know why it works)

This fix is;

<lastmod>
  <xsl:value-of select="@updateDate" />+00:00
</lastmod>

Also umbraco:format-date has been removed, as the date does not need formatting. 

Just thought it should be written down here ;) Laux