Poor performance on local server

[Host-in-the-Shell]

But I'm still very confused about all (Reddit) answers simply stating to turn off transcoding as if there was a clear single option to do that.

While logged in as admin, you can go to your dashboard > users > 3 dot menu on the user you want to disable transcode for > edit user > uncheck under 'media playback' anything related to transcoding > save.

That said, this is more of a workaround and it would be best to correctly troubleshoot the issue by providing logs like others have suggested.