Hello,
There are no performance differences between using invisible or headless options. The performance of LibreOffice is directly related to the performance of the server. We recommend using an SSD disk and a good CPU to get the best performance. Also, please note that complex documents will need more time to be transformed than simple documents.
The native mode is useful when the DOCX contains supported content and styles (https://www.phpdocx.com/documentation/conversion-plugin/preparing-the-templates-for-its-conversion).
On https://www.phpdocx.com/documentation/conversion-plugin/common-problems-and-possible-errors is detailed how to use concurrent conversions.
For further support about this same question, please write an email to contact[at]phpdocx.com. Also, please note the support included in licenses as detailed on https://www.phpdocx.com/support. Your license was purchased more than a year and a half ago, and it doesn't include active LUS, so support is limited to general questions and doubts. We recommend you upgrade to phpdocx 17 and include LUS to use all the new features from phpdocx (https://www.phpdocx.com/news/) and get technical support.
Regards.