The real life senario where I found the problem was with this XSL :

<img src="{resolver:resolveImage('explorer', ametys:resourcesByPath('CMIS')/resource[@name='ametys.jpg']/@id, 400, 400)}"/>

After reducing it, there is already the problem with

<xsl:value-of select="ametys:resourcesByPath('CMIS')" />

When the XML is parsed, there is a network call to the CMIS server on each node