The weird thing is that I'm sure it didn't always do this. I thought it might be because I'm trying to cast with FX_Cast but even with Chrome this also happens. In the past this sometimes happened when you'd expect, if the internet was particularly slow that day for some reason, or if I try to play something with a high frame rate, but in general I could expect HD 1080p quality. While I can't know exactly what quality is casting, it looks more like 480 and sometimes 360.

Don't know if its relevant, but it's a chromecast 1 I've been using since 2014.