Except that you can use Gmail through any IMAP/POP client (https://support.google.com/mail/answer/7126229?hl=en) while that's not possible with Hey where you always have to use their apps.
This is proven by sending an email from one Gmail account to another. It is all but instant. I swear once the email arrived before the Enter key had debounced! SMTP can't move things quite that fast.