The aim is to surround occurences of <row> - tags with a <table> - tag.
I've done this with "good" XSL, what appears to be real slow, and with a
"bad" version, which inserts tags as <xsl:text>.

You didn't show your "good" version, but the standard approach here
would be the identity template with a specific template for the <row>
element (not tag!)

<xsl:template match="@*|node()">
 <xsl:copy>
   <xsl:apply-templates select="@*|node()"/>
 </
</

<xsl:template match="row">
 <table>
   <xsl:apply-templates/>
 </table>
</

I'm pretty confident that would be faster (or at least just-as-fast)
as your current fastest solution.

(your "bad" version is really common amongst casual XSLT'ers when they
get stuck, by the way)

--
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