You have to ask yourself how much effort is really put in to all these Beta tests? Companies keep releasing final builds that are clearly bug riddled and fixes are always just around the corner! My comments are not just pointed in the direction of Nokia, Apple and Samsung are no different. Samsung have gone as far to pull the Oreo Update: https://www.theverge.com/circuitbreaker/2018/2/15/17015952/galaxy-s8-oreo-update-random-reboots-samsung
Every release is a Beta these days and by the time anything starts to become stable, they move on to another major release and we're all back to square one! If it is a hardware issue then nothing is going to fix it. Maybe that's why there are now reports of a 3GB Nokia 5 floating around?
Did you get the update for your nokia 5 to android 8 here in Portugal? Mine doesn't showed me still the update besides what happened to you on the first time.
So today i got the update to Android 8 and got the same problem you got. Started good, but after a few hours it started slowing down that sometimes the apps just froze. Did what you did, a factory reset, and after a lot of work it seems it's working normally.