[xsl] Joining two XML-files, can be a O(n)?

I write a xslt to join two xml files a year ago. But I can not stand
with its low performance.
So I ask for help here.

It was a hack to FreeMind.
If you like to know what mmx_file is, you can follow the following URLs:
   * 
http://freemind.sourceforge.net/wiki/index.php/User:Jiangxin/Patch_save_extra_attributes_outof_mmfile
   * 
http://freemind.sourceforge.net/wiki/index.php/User:Jiangxin/Patch_load_mm_file_with_mmx_file

========== XSLT file: join.xslt ==========
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform";>
   <xsl:output method="xml" version="1.0" encoding="utf-8" indent="yes" />

   <xsl:param name="mmx_file" />

   <xsl:template match="map">
       <map>
           <xsl:copy-of select="@*" />
           <xsl:apply-templates />
       </map>
   </xsl:template>

   <xsl:template match="node">
       <xsl:param name="mmx_node" select="document($mmx_file)" />
       <xsl:copy>
           <xsl:copy-of select="@*" />
           <xsl:copy-of  select="$mmx_node//node[(_at_)ID=current()/@ID]/@*" />
           <xsl:apply-templates />
       </xsl:copy>
   </xsl:template>

   <xsl:template match="*">
     <xsl:copy-of select="."/>
   </xsl:template>

</xsl:stylesheet>

========== XML file 1: x.mm ==========
<?xml version="1.0" encoding="UTF-8"?>
<map version="0.9.0_Beta_8">
<node ID="Freemind_Link_1439916855" TEXT="something">
<node FOLDED="true" ID="_" POSITION="right" TEXT="...">
<node ID="Freemind_Link_1446446787" TEXT="..."/>
<node ID="Freemind_Link_1864715670" TEXT="..."/>
</node>
</node>
... ...
another 8000 nodes!
... ...
</map>

========== XML file 2: x.mmx  ==========
<?xml version="1.0" encoding="UTF-8"?>
<map version="0.9.0_Beta_8">
<node CREATED="1173523728454" ID="Freemind_Link_1439916855"
MODIFIED="1173523728454">
<node FOLDED="FALSE" CREATED="1173523728455" ID="_" MODIFIED="1173523881485">
<node CREATED="1173523728456" ID="Freemind_Link_1446446787"
MODIFIED="1173523888376"/>
<node CREATED="1173523728457" ID="Freemind_Link_1864715670"
MODIFIED="1173523894471"/>
</node>
<node CREATED="1173523728458" ID="Freemind_Link_1476641610"
MODIFIED="1173523728458"/>
</node>
... ...
another 8000 nodes!
... ...
</map>

========== xsltproc test  ==========
when operate on large XML file(contain more then 8000 nodes), it will
cost 8 minites!!!
# time  xsltproc  --stringparam mmx_file x.mmx  join.xslt  x.mm > /dev/null

real    8m7.242s
user    7m48.237s
sys     0m0.084s

========== O(n^2) ==========
I know the problem is the process is a o(n^2).
   <xsl:param name="mmx_node" select="document($mmx_file)" />

I want to know whether there is a solution in xslt scope?

Thanks.

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