The examples of large (more than 10^6 lines) DBF databses can be taken here: http://www.gnivc.ru/html/gnivcsoft/KLADR/Base.7z When opening them in LibreOffice, the DBF file format is imported into Libreoffice, what takes a really long time and is done in one thread only. When saved to ODS after importing, the files are opened quickly. Maybe make a multithread import?
Closing as wontfix like bug 101746.