To speed up opening & saving attachments, try doing the below from @Gary post from the following thread.
https://forum.emclient.com/t/slow-attachment-downloads-since-9-2-update/86255/17
(Quote Extract from Garys Post)
“There is something you can try. It won’t speed up the connection or server, but it will cache attachments when they arrive so when you later click on them, they should open immediately”.
“Go to Menu > Accounts and click on the IMAP or Exchange tab for the account. Scroll down to Sync Options, tick both of them, and click on Save & Close”.
(Gmail IMAP example sync options enabled)
