Hello,
Please note that the trial package adds extra information and contents (background, trial watermarks, and extra metadata) to the DOCX output than the purchase licenses, so it may be related.
All packages are fully tested with thousands of HTML contents added without issues when opening the DOCX using MS Word 2003 to MS Word 2019. We have also tested your code with Basic/Advanced/Premium licenses and MS Word 2010, MS Word 2013, MS Word 2016 and LibreOffice 6 and there's no high CPU/memory usage when opening the DOCX and we can select and change the document perfectly.
The trial package doesn't include this kind of support (we'd need to check your DOCX output to compare it with the one we get, maybe some extra content is being added), but don't worry, although we can't replicate the error if after purchasing a license you still find the same issue the dev team will help you to solve it.
Regards.