At the top of this forum is a 'Sticky' thread, titled
Cleaning up Text Pasted from Websites, E-mails, PDFs etc., for cleaning up documents such as you describe:
https://www.msofficeforums.com/word/...s-e-mails.html
In all likelihood the delays you're experiencing are due to Word having to repaginate the document every time you delete a paragraph break. Switching to Draft view and switching off background repagination may help.