mac os 10.10.3 mail.app typing lag issue

Fix:

1. Open a message-editing window. Under the Edit menu, deselect all the options in the Spelling and Substitutions  sub-menus. Test.

2. Many problems with Mail are caused by iCloud Drive. If you use the service, open the iCloud preference pane, click Options... next to the iCloud Drive box, and in the sheet that opens, uncheck the box marked Mail. See whether there's an improvement.

System wide:  Go to System Preferences > Extensions > Actions and untick the "Markup" box

Mail only:  Customize the new-message toolbar (open a new message, right-click/control-click on the tool bar, select "Customize Toolbar" and add or subtract something - especially the "Markup" icon, but changing anything may work)