If you stream 10 video files at the same time, is that perhaps just a limit for a rapsberry pi?
Do you see a iowait on top (just in case: https://bencane.com/2012/08/06/troubleshooting-high-io-wait-in-linux/)?
What I found is when you use encryption (I have a high CPU usage when watching a video via the browser) or external storage (Video playback is slowly raising the CPU load and also releases cpu ressources slowly after the video has stopped · Issue #487 · nextcloud/viewer · GitHub).