Netflix can literally implement fast.com to do the exact same operation as when streaming and then measure performance of the stream. Yes you could do a timer,

Netflix can literally implement fast.com to do the exact same operation as when streaming and then measure performance of the stream. Yes you could do a timer, but that would mean e.g. first 30s of all Netflix streams to be much faster and then slow down just in case they were in fact speed tests. Also would be difficult to differentiate between running a speed test vs a stream that gets its connection closed and reconnects. All of this is an order of magnitude for ISPs to deal with than unthrottling traffic for specific domains. It doesn’t have to be perfect, but making it prohibitively more expensive helps a lot.