It requires the following features: p:http-request or timeout-support.
The pipeline
<p:declare-step xmlns:c="http://www.w3.org/ns/xproc-step" xmlns:err="http://www.w3.org/ns/xproc-error" xmlns:p="http://www.w3.org/ns/xproc" xmlns:xs="http://www.w3.org/2001/XMLSchema" exclude-inline-prefixes="c xs" version="3.0">
<p:output port="result"/>
<p:option name="WHOST" select="'http://localhost:8246'" static="true"/>
<p:for-each timeout="2">
<p:with-input>
<doc/>
</p:with-input>
<p:http-request method="get" href="{$WHOST}/service/slow" assert="true()">
<p:with-input>
<p:empty/>
</p:with-input>
</p:http-request>
<!-- Nothing else should get run, but I can't think
of an interoperable way to test that... -->
<p:identity/>
</p:for-each>
</p:declare-step>
