Have the same issue. Only three apps allowed in the background on Nokia 5.
Restart the phone every day or 2. No other way with Nokia. They do not care about us :)
How many apps do you have installed? -> Play Store, My Apps, Installed: (number) shown
I now own a Nokia 7 Plus, too, and as this one has 1 GB more (and faster) RAM it is always lightning fast. My Nokia 5 became faster after I uninstalled a lot of installed apps. My 7+ currently has 169 apps installed.
Oliver, it's a special problem with Nokia 5. They actually admitted it on Twitter, but no-one even tried to fix it. To be honest, no problems are fixed with this company unless it makes headlines. That's why I will never-ever buy any Nokia device again. Too bad I'm stuck with this crap for few upcoming years now.
This same issue affects Nokia 6 2017 as well. The ONLY REAL working solution is to reboot every day or 2. I've heard that new phones from HMD don't have this issue but I have no personal experience with them.
Well.. You can roll back to 7.1.1 if you want to. That helps. But Oreo has too many good functions that Nougat didn't have.