How to improve splitting xml file performance

Here's my suggestion. It requires a streaming XSLT 3.0 processor: which means in practice that it needs Saxon-EE 9.3.

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="3.0">

<xsl:mode streamable="yes">

<xsl:template match="/">
  <xsl:apply-templates select="company/staff"/>
</xsl:template>

<xsl:template match=staff">
  <xsl:variable name="v" as="element(staff)">
    <xsl:copy-of select="."/>
  </xsl:variable>
  <xsl:if test="$v/nickname">
    <xsl:result-document href="{@id}.xml">
      <xsl:copy-of select="$v"/>
    </xsl:result-document>
  </xsl:if>
</xsl:template>

</xsl:stylesheet>

In practice, though, unless you have hundreds of megabytes of data, I suspect a non-streaming solution will be quite fast enough, and probably faster than your hand-written Java code, given that your Java code is nothing to get excited about. At any rate, give an XSLT solution a try before you write reams of low-level Java. It's a routine problem, after all.

You could do the following with StAX:

Algorithm

1. Read and hold onto the root element event.
2. Read first chunk of XML:
   1. Queue events until condition has been met.
   2. If condition has been met:
      1. Write start document event.
      2. Write out root start element event
      3. Write out split start element event
      4. Write out queued events
      5. Write out remaining events for this section.
   3. If condition was not met then do nothing.
3. Repeat step 2 with next chunk of XML

Code for Your Use Case

The following code uses StAX APIs to break up the document as outlined in your question:

package forum7408938;

import java.io.*;
import java.util.*;

import javax.xml.namespace.QName;
import javax.xml.stream.*;
import javax.xml.stream.events.*;

public class Demo {

    public static void main(String[] args) throws Exception  {
        Demo demo = new Demo();
        demo.split("src/forum7408938/input.xml", "nickname");
        //demo.split("src/forum7408938/input.xml", null);
    }

    private void split(String xmlResource, String condition) throws Exception {
        XMLEventFactory xef = XMLEventFactory.newFactory();
        XMLInputFactory xif = XMLInputFactory.newInstance();
        XMLEventReader xer = xif.createXMLEventReader(new FileReader(xmlResource));
        StartElement rootStartElement = xer.nextTag().asStartElement(); // Advance to statements element
        StartDocument startDocument = xef.createStartDocument();
        EndDocument endDocument = xef.createEndDocument();

        XMLOutputFactory xof = XMLOutputFactory.newFactory();
        while(xer.hasNext() && !xer.peek().isEndDocument()) {
            boolean metCondition;
            XMLEvent xmlEvent = xer.nextTag();
            if(!xmlEvent.isStartElement()) {
                break;
            }
            // BOUNTY CRITERIA