Catalina Mail super slow when composing new emails?
Open up a Finder app session
You will need access to your User library
Go to Folder Containers &gt; com.apple.mail &gt; Data &gt; Library &gt; Saved Application State

Go &gt; Go To Folder and type in as shown above.
Move com.apple.mail.savedState to Trash

Move this folder to the trash
Next, in Finder app, press down the Option key and click on Go at the top of the menu
Type in ~/Library/Containers
Drag com.apple.mail and com.apple.MailServiceAgent out to your desktop
Similarly, remove the envelope files from ~/Library/Mail/MailData and drag them to trash


Envelope Index
Envelope Index-shm
Envelope Index-wal

Empty your Trash and Restart your Mac


If your Mail works without hiccups, you can move the files from your desktop into Trash ( file from step 8 above).