It takes at least 4-5 seconds to open the email.
If I’d just click into the eM Client’s main window, I could read the email immediately
If you are reading emails “remotely” like with an eg: IMAP or Exchange account, the email can take a little time to load the first time. So 4-5 secs on average is normal as @John_A advised.
The next time you click on that same email, it should then be cached and load almost instantly.
However you can alternatively set the account in Sync Options to “allways download messages offline including attachments etc”. See my post in the below thread. It does take up more mail database space, but will speed up that process.