There is a delay between notification sound and notification banner for mail app in High Sierra. How can I fix this?
Had the same issue on a 2017 27" iMac.
Spoke to Apple support....(I have APPLE CARE)

They had me do 3 things:


Upgrade to the latest version of macOS High Sierra 10.13.4 (That alone did not fix it).
Reset the SMC (for my desktop machine, they had me shut down and unplug the machine for 15 seconds instead).
Reset the NVRAM.