Why is there a delay between mail notification sound and notification banner?

I have a MBPR 13" running on High Sierra. The problem is that everytime I receive a email on the mail app, there is a delay between the notification sound and notification banner i.e. I would hear a notification beep and 4-5 later, the notification banner would appear on screen. Is there a fix to this problem? I have "turning it off and on" and other fixes but it doesn't seem to work. I should add that the problem started after I updated to High Sierra a couple months ago!