This happens to me when I accidentally click on the header/footer areas, summoning the blue tabs. Clicking on the + to get the dropdown menu to delete it, letting the text settle, then saving and reopening the document seems to be a workaround, but is terribly inconvenient.
Bug filed here: https://bugs.documentfoundation.org/show_bug.cgi?id=152019#c1
Bug post contains a response that solved the header/footer appearance issue forever, (You can disable the feature if it is not needed to you. It is configurable under Options->LibreOffice->Advanced->Open Expert Configuration; look for UseHeaderFooterMenu) and stopped the slow-highlighting issue.