That parameter string that @Gary posted in his original link seemed to work fine for me for large attachments “when i tested it” on my Yahoo, Gmail and Outlook IMAP accounts with the latest V9.2 for Pc & Mac. eg: I downloaded a 17+MB .mp4 video attachment (not locally cached) within a few seconds in eM Client on my 50mbps nbn connection. The video opened and ran all ok.
Note:- Be aware though using the raw string parameter “bypasses the normal reliable chunk style download verification method” eM Client uses by default. So yes it downloads quick, but you could possibly get a downloaded file that might not always be 100% reliable.