I'm beginning to think that perhaps my performance hopes were a bit too inflated, given the size and complexity of our database. After a fresh optimization, and with -Xms2g -Xmx10g, the following query takes 1492ms:

declare namespace ernm="http://ddex.net/xml/ern/411";
for $r in /ernm:NewReleaseMessage  
  for $track_release in $r/ReleaseList/TrackRelease
    return
      <identity>
        <isrc>{ $track_release/ReleaseId/ISRC/text() }</isrc>
      </identity>

When I add a little bit more to the query, we go up to 11204ms:

declare namespace ernm="http://ddex.net/xml/ern/411";
for $r in /ernm:NewReleaseMessage
  let $parties := $r/PartyList/Party
  for $track_release in $r/ReleaseList/TrackRelease  
    let $rlr := $track_release/ReleaseLabelReference/text()  
    let $party := $parties[PartyReference/text() = $rlr]
    return
      <identity>
        <isrc>{ $track_release/ReleaseId/ISRC/text() }</isrc>
        <sublabel>{ $party/PartyName/FullName/text() }</sublabel>
      </identity>

The fastest full query that I've been able to make so far takes 70450ms:

declare namespace ernm="http://ddex.net/xml/ern/411";
for $r in /ernm:NewReleaseMessage
  let $parties := $r/PartyList/Party
  let $sound_recordings := $r/ResourceList/SoundRecording
  let $releases := $r/ReleaseList/Release

  for $track_release in $r/ReleaseList/TrackRelease  
    let $rrr := $track_release/ReleaseResourceReference/text()
    let $rlr := $track_release/ReleaseLabelReference/text()

    let $sound_recording := $sound_recordings[ResourceReference/text() = $rrr]
    let $release := $releases[ResourceGroup/ResourceGroup/ResourceGroupContentItem/ReleaseResourceReference/text() = $rrr]
    let $party := $parties[PartyReference/text() = $rlr]
    return
      <identity>
        <isrc>{ $track_release/ReleaseId/ISRC/text() }</isrc>
        <artist>{ fn:string-join($sound_recording/DisplayArtistName/text(), '/') }</artist>
        <title>{ $sound_recording/DisplayTitleText/text() }</title>
        <album>{ $release/DisplayTitleText/text() }</album>
        <icpn>{ $release/ReleaseId/ICPN/text() }</icpn>
        <sublabel>{ $party/PartyName/FullName/text() }</sublabel>
      </identity>