This isn't so much the normal "slow to update", i.e. messages or notifications don't always come through instantly, this is more about the messages or notifications not clearing if read on another device.
I used to have a Huawei P9, was a great phone but it started getting a bit slow and the bluetooth range was limiting, I opted for a refurb S8, which is great except for the notifications issue. I've read and read about it, but people only seem to be saying about the apps being slow to notify, and to be honest, mine in general is good to notify me from Whatsapp or messenger, but my Huawei would also notify me instantly when my partner had played her word in Words with Friends 2. The Galaxy? could be 2 minutes, could be 12 hours.
I tried the battery optimisation fix as listed elsewhere, but it was already enabled.
Anyway, I can do with slow notifications, but, and I don't know why, what really grinds me is if I read a message on PC, that my phone still shows it as unread, unless I go into messenger. My Huawei would recognise it was read and remove the bubble notification so I didn't think, ooh, another few messages from the boys. I'm just waiting to see how long it takes to realize, I am up to 1 hour now and my phone is still telling me I have new messages when I don't.