I have a Spark 100/20 connection with ATV4 connected via Ethernet. Prior to the new YouTube app being rolled out I had no quality issues with streaming from YouTube and still have no issues with OTHER APPS.
However ever since the new-look YouTube app was rolled out I have noticed very low bitrate streaming. The same videos on the YouTube app on my LG TV stream in 4K almost instantly (yes I know 1080p is the most the ATV4 will do but most time I am stuck on 360p).
Here are some links to photos of the “stats for nerds” showing for the same video in ATV4 in 360p versus LG in 4K if anyone can determine anything from them:
ATV4 @ 360p: https://drive.google.com/open?id=1gEkhDkf8qE5ohB0wSRGnuA8XMfB5HUpm
LG @ 4K: https://drive.google.com/open?id=1FQZXuW59DPkUau1iPGXslAy6DHUSBTEf
Anybody else seeing this or have any idea what might be going on?