I'm having a similar issue, at least when casting to my Channel Master Stream+ (which runs Android TV.) It'll actually start as a high-quality HD stream, then scale down to what appears to be a 480p-quality stream after a couple of seconds. I have a gigabit connection which speedtests and real-world gives much of that gigabit, and I've tried both 5GHz wireless and wired-in for my Stream+. I don't have this issue with other apps on the Stream+.
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