2.A clean desktop.
The items in your desktop folder actually use system resources in more than one place including needing to be rendered as little images perpetually on your screen. There are many way to access files easily. Even if you put one folder on the desktop and drag them all into it, it will help. Even better, go into your finder window and move that “desktop clutter folder” to Documents, then drag the folder icon to the right side of your dock. You will then have a shortcut to it. Ok done.
To keep from having slow email problems that many of us experienced in the past, and other difficulties; if you use Apple Mail or any other email software, each of your email accounts should be on the more modern, easier to sync IMAP protocol. Check with your email provider if you are not sure.