XSL performance with key from external xml doc.

I have been using an XSL profiler ( catchXSL ) and notice that I have a
significant performance bottle neck
in looking up keys from an external XML document.

        lookUpDoc is initialized as:
            <xsl:variable name = "lookUpDoc" select =
"document('schemes.xml')"/>
        Code Snippet:
                <xsl:for-each select = "$lookUpDoc">
                        <xsl:variable name = "tagValue" select =
"key($schemeName,$normalizedTag)"/>
                </xsl:for-each>

This lookup is called too often. Is there any faster way to achieve this ?.

thanks,
Ashish.

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