Mail on OS 12.6.7 very slow to load message content

Hi James Harrell,

Thank you for using the Apple Support Communities! For this issue, we would recommend if using an IMAP or Exchange account that you remove and re-add the account by following the steps below:

Remove email accounts in Mail on Mac

1. In the Mail app  on your Mac, choose Mail > Preferences, then click Accounts. 

2. Select an account, then click the Remove button .

Note: If the account is used by other apps on your Mac, you’re asked to remove the account in Internet Accounts System Preferences. Click the button to open Internet Accounts, then deselect the Mail checkbox for the account. To stop using the account in all apps, make sure the account is selected, then click the Remove button 

Add email accounts in Mail on Mac

1. In Mail, choose Mail > Add Account, select an account type, then enter your account information. Make sure the Mail checkbox is selected for the account.

2. If you’re using an account on your Mac with other apps, such as Contacts or Messages, you can also use that account with Mail. In Mail, choose Mail > Accounts to open Internet Accounts settings, then click Add Account on the right.

3. You can set options for your email accounts in Mail settings.

If the issue remains, we recommend that you reach out to Apple Support directly to take a further look. Please reach out here:

Apple Support

Hope this helps!

Have a great day.