Microsoft is claiming that they fixed "serious performance issues" with RTF files in Version 2108 (Build 14326.20738), January 11 (Release notes for Semi-Annual Enterprise Channel releases - Office release notes | Microsoft Docs). If you're not in IT, you might ask your IT dept. to submit a ticket to MSFT. You could also try rolling back to an earlier version, but that's a bit of a pain. Also, what happens if you save the file as a .docx, make revisions, then save as .rtf?