I hadn't noticed LibreOffice lags any worse than MS-office. But, if you haven't tried, for anything bigger than a chapter or so, do yourself a favour and try a

I hadn't noticed LibreOffice lags any worse than MS-office. But, if you haven't tried, for anything bigger than a chapter or so, do yourself a favour and try a "proper" typesetting system - LaTeX, SILE, or the like. There will be a learning curve but I find my attention stays on the content, as I'm not continually distracted by the format issues that are better delayed until the text is complete. You'll want a reasonable text editor with spell checker, preferably one that knows to ignore formatting commands. Note you will end up with PDF output - if you really have to submit docx, my condolences!