Mac Mail SLOW to load content

Hello woodyfromdewitt,

We would like to see how we can help you with the Mail app loading slowly. If you test the issue in a new user, does the issue persist? This will help us determine what may be the cause of those emails loading slowly. Follow the steps in this link to create a new User: Add a user or group on Mac

"Add a user

1. On your Mac, choose Apple menu  > System Settings, then click Users & Groups  in the sidebar. (You may need to scroll down.)
2. Click the Add Account button below the list of users on the right (you may be asked to enter your password.)
3. Click the New Account pop-up menu, then choose a type of user.

• Administrator: An administrator can add and manage other users, install apps, and change settings. The new user you create when you first set up your Mac is an administrator. Your Mac can have multiple administrators. You can create new ones, and convert standard users to administrators. Don’t set up automatic login for an administrator. If you do, someone could simply restart your Mac and gain access with administrator privileges. To keep your Mac secure, don’t share administrator names and passwords.
• Standard: Standard users are set up by an administrator. Standard users can install apps and change their own settings, but can’t add other users or change other users’ settings.
• Sharing Only: Sharing-only users can access shared files remotely, but can’t log in to the computer or change settings. To give the user permission to access your shared files or screen, you may need to change options in File Sharing, Screen Sharing, or Remote Management settings. See Set up file sharing and Share the screen of another Mac.

1. For more information about the options for each type of user, click the Help button in the lower-left corner of the dialog.
2. Enter a full name for the new user. An account name is generated automatically. To use a different account name, enter it now—you can’t change it later.
3. Enter a password for the user, then enter it again to verify. Enter a password hint to help the user remember their password.
4. Click Create User."

Cheers!

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Sometimes the Favorites get lost in upgrades or they get hidden:

Or, you an repopulate them by clicking on the "+" in a circle button and add the various All mailboxes you want in the Favorites section.