If you use the Mail app on your Mac and notice that it seems to have slowed down, you can try rebuilding the message index databases to see if that will help. The more email addresses and the more mail you save, the more likely this is to assist with a slow system. It might take a while to complete, but your email will still work perfectly so it is worth doing periodically.
With the Mail app open, Select Mailbox > Rebuild.
If you are in the middle of rebuilding and you have to quit out of Mail for some reason, don’t fret. The rebuilding will take up where it left off when you restart mail.