fast.com checks the speed to Netflix servers. So if they juice that, they also have to juice Netflix streaming speed unless they've figured out a clever way to fingerprint just fast.com traffic over HTTPS.
I'd assume a legitimate Netflix watching session looks pretty different from a speed test metadata wise. Small amounts of data sent for a bit while you're finding something to watch, then a consistent rate while you're streaming it, as opposed to an immediate maxing out of the connection for 45 seconds.