correct. I have now uninstalled it and all is well. Do not dare to install it 
but if external access was fine without performance degradation, the issue was/is somewhere inside your network. How do you access the system locally? Did you used the same DDNS? Or local IP?
i use the local ip. Have now installed Nextcloud again. The problem still exists. I give up
When it’s working fast from the internet and slow when you try to reach it by LAN IP, then it’s not a problem of nextcloud but a problem with your LAN. (What is weird, the most time it’s the other way around: good from LAN and bad from external)
How do you connect from the internet? Via VPN or DDNS?
As said about ddns from the fritbox. I also find strange because I have also replaced the lan cable and still the problem persists.
How does it behaves if using DDNS from inside your network?
Runs great. I might have to completely clean up Nextcloud from my dietpi and completely reinstall?
Ok. I’m at work right now. As soon as I’m home I’ll take a look and get back to you. Thanks for the info.
I’m not sure that a reinstall will solve this because internal as well as external access is working fine using DDNS name. Means, the issue is with local IP access only.
Are you using HTTPS/SSL for DDNS access (intern + extern)?
On local access, you use HTTP only? Without SSL? If yes, how does it behave if you go for HTTPS on local IP? (ignoring SSL certificate warnings)
The app is not installed. Nothing has been changed in the settings and apps have been added. The only app I had installed was the external hard drive.