Populating keys with values from other documents

I am trying to look up a value in an XML document (people.xml) based
on a @ref attribute in my primary document (source.xml).  I know that
the @ref values in source.xml will match the @xml:id values in
people.xml.  I can do something like this:

<xsl:template match="name">
        <xsl:variable name="uriRef">
            <xsl:value-of select="(ancestor::*[@xml:id]/@xml:id)[last()]"/>
        </xsl:variable>
        <xsl:variable name="person"
            select="document('file:///people.xml')//person[@xml:id=concat($var,
'')]"/>
        <xsl:if test="$person/residence/country">
        <span about="ont:place#{$uriRef}" property="foaf:based_near" >
            <xsl:value-of select="$person/residence/country"/>
        </span>
        </xsl:if>
</xsl:template>

But this is incredibly slow for large documents.  I have experimented
with using xsl:key, but I haven't had much.  Essentially I would like
to pre-populate the key with the values from people.xml, but I can't
quite figure out how to do that in a way that doesn't get me into a
looping scenario and/or that allows me to pass my $uriRef value to the
template that processes the key.  Can anyone explain a better
approach?

Thanks, Andy