[Been with Apple since the early 90's and] I have the same issue with a brand new Mac Studio, macOS Sonoma, same problem on IOS17 - so it's not the hardware, it's this clunky, outdated, Apple Mail program.
Takes 5mins to download and preview a 50kb file - what the actual _ _ _ _!
*Here's a radical suggestion, try separating all the attachments into it's own area, instead of continually burying them in endless conversation threads – just a thought.
Given the previous complaint was back in August, I'm not expecting a solution anytime soon...