It may be a slow connection to the server, or just a slow server.
There is something you can try though. It won’t speed up the connection or server, but it will cache messages and attachments when they arrive so when you later click on them, they should open immediately.
Go to Menu > Accounts and click on the IMAP tab for the account. Scroll down to Sync Options, tick both of them, and click on Save & Close.
This is from another of your posts, where I see you don’t have the options ticked.
