In addtion to gizmo2501’s list, here are some other threads on the same topic:
Slow to download attachments - Mail - eM Client
Very Slow Download of Video Attachments - Mail - eM Client
Yahoo mail downloading attachments: super slow - Mail - eM Client
You have to read through these threads to find solutions
In a July 2023 post in one of these threads, Gary recommended adding a string parameter to the advanced options box.
He said this downloads attachments in a way that is faster but less reliable, and so it should only be used if you were having a problem.
In a September 2023 post, he pointed out the checkboxes for downloading attachments in Accounts > IMAP > Sync Options.
My understanding is that this basically pre-downloads all the attachments you get locally. (It would be great if those messages and their attachments go back to the server when you move them out of the inbox, but IDK.)
So, those are the two fixes on offer now in one post. I tested both of them and they worked.