If disabling your anti-virus, VPN, proxy or firewall, as well as restarting your router does not help, then it may be a slow connection to the server elsewhere, 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 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.