Mail on Mac loads very slow

Hey there d18ge,

If the issue continues, test in a new Administrator account to further isolate the cause of this. Here’s how:

1. On your Mac, choose Apple menu > System Preferences, then click Users & Groups. If the lock at the bottom left is locked, click it to unlock the preference pane.
2. Click the Add button below the list of users.
3. Click the New Account pop-up menu, then choose Administrator.
4. 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.
5. Enter a password for the user, then enter it again to verify. Enter a password hint to help the user remember their password.
6. Click Create User.

You can read more here:

Set up users, guests, and groups on Mac

Take care.

Thanks for posting in the Apple Support Communities d18ge,

There's a lot of factors to consider when determining the cause for email messages downloading or syncing slower than you'd expect them to.

The following page offers some general guidance on the subject of performance, that offers a few relevant suggestions for isolating the cause. If your Mac runs slowly

•Your computer’s startup disk may not have enough free disk space. To make disk space available, you can move files to another disk or an external storage device, then delete files you no longer need on the startup disk. macOS can also help you optimize storage by storing files in the cloud and helping you identify files you no longer need.

•An app you’re using may require more memory than your Mac has easily available. To see how much memory your Mac has, choose Apple menu > About This Mac, then check the amount of memory listed in the Overview pane. See the documentation that came with the app to find out its system requirements.

To find out how to increase your computer’s memory, choose Apple menu > About This Mac, click Memory, then click Memory Upgrade Instructions in the lower-right corner. (Not available on all computers.)

•Quit any app that isn’t compatible with your Mac. For example, an app may require a different processor or graphics card. See the documentation that came with the app for information about its system requirements.

-To see what processor your computer has, choose Apple menu > About This Mac, then click Overview.

-To see what graphics card your computer has, choose Apple menu > About This Mac, then click Displays. The name of the graphics card in your computer is displayed in the middle of the window.

•Quit any apps you’re not using.

•If you have problems with your disk, use Disk Utility to verify and try and fix any issues.

•Use Activity Monitor to see if a particular app or activity is making a large demand on your Mac.

You can find guidance on using Activity Monitor, here: Activity Monitor User Guide

We hope this helps!