Until I found out more about the later mentioned validator build on
Saxon, my XSLT related question is this:

Is the best (performant) way to work on the selection the following?

<xsl:variable name="precedingStarts" select="count(preceding::text:start)"/>

<xsl:if test="$precedingStarts != 0">
    <xsl:variable name="followingEnds"
select="count(following::text:end)"/>
    <xsl:if test="$precedingStarts = $followingEnds">
    <!-- do something -->
    </xsl:if>
</xsl:if>

don't know, though it smells like you could just work off some
alternating thing as in theory starts and ends should match (and
alternate), should save xslt from calculating a few count() operations
each time.

its like the alternate row colour problem for tables, e.g. (position()
- 1) mod 2

irregardless of how you are keying your text: element(s) where just
remember that your 1st text:element to 2nd text:element would be your
first range, 3rd text:element to 4th text:element be your second
range, etc...

Is there any possibility to cache this?

there is a possibility to compile your xslt using xsltc, I think
latest saxon supports this.