Notifications are very delayed on android - but only sometimes (apparently FIXED) I searched for my problem but what I found either wasn't answered or was very old response and maybe things have changed since then...

I just got a Samsung Galaxy S8 and it's amazing, but maybe since yesterday notifications are a bit slow. I just answered and email that I got an hour ago from my computer while my phone still didn't show the notification. I got some test messages and again, they showed up but only after I opened up gmail app and refreshed it.

What might help find the answer - right now I'm spending most of the time on my computer when I have gmail website opened (for hangouts) and thunderbird for emails. However the days before I was mostly outside with my laptop either turned off of inactive.

Btw, I had a similar issue with the old phone (also android) but I just blamed it being 4 years old and acting up.

So what could be the possible issue? Some kind of setting on my phone? Possible network changes? (I changed cities in those days, maybe the network here sucks and phone tries to use it instead of wifi... I'm just looking for options here). Or is it just the fact that I have gmail / thunderbird opened?

Thanks in advance.
One reason could be you have auto-sync turned of which is resulting in emails not being synced automatically.To enable this
Goto Setfings->Accounts &amp; Sync ->tick Auto-Sync 
also select your google account from there and make sure "Sync Gmail" option is ticked. 
Another reason that may cause this can be battery savers. 
Try turning of "battery saving mode" on your device and test again also turn off any 3rd party battery savers if you have any or white list the Gmail app in them.