Filtering XML to improve performance

Sorry if this is a FAQ- please point me in the right direction if so.

Let's say my xml is this:

<root>
	<items>
		<onlyImportantItem>THIS</onlyImportantItem>
		<junkItem>foo</junkItem>
		<moreJunk>foo</moreJunk>
		<stillMoreJunk>foo</stillMoreJunk>
	</items>
	<items>
		<onlyImportantItem>THAT</onlyImportantItem>
		<junkItem>foo</junkItem>
		<moreJunk>foo</moreJunk>
		<stillMoreJunk>foo</stillMoreJunk>
	</items>
	...
	...
	...
</root>


If I only mainly want to work with <onlyImportantItems>, is there a way that
I can filter out all the other data I don't need when I do the transform?

So for instance, I don't want the XML parser searching through all the
garbage, I want it cut down to just the data I need.  Can I do something
like this at the beginning of all my xsl sheets?

Thanks.