Why would you blame your smartphone for having a slow internet connection maybe it's your IP
to judge it more information is needed.
Nougat or Oreo?
WiFi and mobile services?
3G and 4G?
Have you tried to disable 4G and use it via 3G? In some areas network coverage is poor so it is needed to change from 3G to 4G or vise versa.
As my actual contract doesn't include 4G I'm just on 3G but the Nokia 8 is faster than the Lumia 950 I had before.
I had some strange slow downs on 8.0, but since updating to 8.1 beta I have not encountered them so far.