Netflix constantly degrades for me despite a 100MB/s fiber connection.
Could be ISP side peering or throttling issue, I have had speeds half that with no issues. You can have gigabit speeds and still have buffering issues if there's a peering or throttling issue. YouTube is another one that some ISPs like to throttle/have peering issues with. Try a VPN you will know if your ISP is the reason for it.
I'm sure Apple will run into the same issue with the likes of Comcast when the traffic gets high enough forcing them to pay for better peering deals.
I believe they try to keep bit rate down to a certain level so people don't end up burning through their usage limits. Comcast for me is 1TB, never use it all up but, I'm sure other peoples usage patterns and family sizes may make it a tough balance.
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