I’ve found that zpRetrieveViaShortcode in js/zotpress.shortcode.intext.js executes twice for the update parameter.

I’m not 100% confident of the usefulness of this parameter, because I’ve tried to remove it and it doesn’t break any functionality.

The fact is that this double execution makes the site load slightly slower.

Example URLs by this issue:

https://example.com/wp-admin/admin-ajax.php?action=zpRetrieveViaShortcode&instance_id=zp-InTextBib-zotpress-a9a3978972cef9429be7ea56ffbd68b4&type=intext&item_key=%7B4712101%3A3VUDQ4YH%7D&style=apa&title=no&showimage=false&showtags=false&downloadable=false&shownotes=false&showabstracts=false&citeable=false&target=false&urlwrap=false&highlight=false&sortby=default&order=asc&update=false&request_start=0&request_last=0&zpShortcode_nonce=9963472ca9

https://example.com/wp-admin/admin-ajax.php?action=zpRetrieveViaShortcode&instance_id=zp-InTextBib-zotpress-a9a3978972cef9429be7ea56ffbd68b4&type=intext&item_key=%7B4712101%3A3VUDQ4YH%7D&style=apa&title=no&showimage=false&showtags=false&downloadable=false&shownotes=false&showabstracts=false&citeable=false&target=false&urlwrap=false&highlight=false&sortby=default&order=asc&update=true&request_start=0&request_last=0&zpShortcode_nonce=9963472ca9

If you want to check the differences with the GTMetrix reports:
With the update parameter: https://gtmetrix.com/reports/www.forotoc.com/fXNV23Gp
Without the update parameter: https://gtmetrix.com/reports/www.forotoc.com/VYje2AfQ

Check in the tab “Waterfall” the extra 911ms for the second request.