I think youtube changed something or yt-dlp changed how it handle formats now.
I have this in my mpv.conf
ytdl-format=480p/best[height<=480][ext=mp4]/best
But it stopped working, after playing around I got it to work again with this
ytdl-format=bestvideo[height<=?480]+bestaudio/bestvideo[height<=?480]+bestaudio/best
But why does it feel slower when loading the video?