Rainbow spinning wheel appears often in Mail significantly delaying activities

Rainbow spinning wheel appears often in Mail while writing email, clicking around to correct or edit text, and clicking between email messages. It takes several seconds to stop, and then reappears after a kew more keystrokes. When writing it will appear but the keystrokes entered when frozen do catch up. However if I need to click somewhere I have to wait. This happens very often through composing an email. It has persisted over at least three operating system upgrades that I can recall. Does not appear on most other apps. No issues typing this message for instance. Any advice? Help is greatly appreciated!