Nextcloudpi, Slow Connection in network

Slow connection

Hey Nextcloud Users,

I have a Problem with my nextcloudpi on my Raspberry Pi4. Everything works and is connected, i have only Pictures in it. And i have access with the Android app.
But my Problem is the Speed. Local with LAN and with Wlan and the App. The Folders and the server loads fast, but the thumbnails or the whole Picture takes very long to load. Also with Pc that is connected Local Lan.
I use the IP to go in the Cloud.
Thats really hard, but i can’t find a solution.

The Pi 4 is 4Gb Ram and is Connected with a 1gbit Lan.
The Pc Also 1Gbit Lan.
Wlan 5ghz and 2,4ghz from Smartphone App.
Hdd is a External WD elements portable 2,5" USB 3.0.
My Internetconnection is 80mbit, with cloud online 50Mbit. Also should be enough.

I would unterstand if the speed is slow from the outside. But slow also in own network, i dont really understand the problem.