Typing in mail is delayed, also delay in loading received emails

For the last several weeks, while typing in Mac Mail, there is often a frustrating lag from the typing on the keyboard to the screen showing the letters typed. It can be about 10 seconds. It does not happen all the time, but at least daily. In addtition, when I receive an alert that I have a new email, and then go to open that email, it takes up to 20 seconds for the email to load.

This slowness does not happen in any other application on my imac, just mail. I have 8 GB of memory which should be plenty. Activity monitor isn't showing anything unusual other than disk activity spikes.

Any thoughts?