yeah i noticed that with mpv as well. if you pass it a m3u with multiple sources (eg, same show but at different quality) it seems to randomly (iirc) select a source, which takes a very long time to do so. after the source is selected, it loads instantly, so that part of the m3u loading is okay.
but yeah, something to parse the multiple sources, list them out to the user, showing the quality difference (if listed in the m3u master) would be a nice-to-have feature. this can also be used for crunchyroll and funimation