speedof.me is also vastly superior to Ookla's speedtest.net.
And as others pointed out, it doesn't even need to be a case of them gaming/prioritizing speed tests. Generally that is simple HTTP traffic - you can see for yourself by downloading some Linux ISO from a webpage (HTTP/HTTPS). You might need to try a few but generally traffic is shaped/prioritized based on protocol, not specific destination (save for video services, which while served over HTTPS/443, always comes from specific content delivery networks (CDNs). While a lot of internet traffic comes from CDNs, Netflix and Youtube and others all operate their own CDNs, so when throttling/prioritizing occurs on specific content that shares a protocol with just about everything else on the web, they target CDNs to throttle traffic.
So if you just go and find random but major operating system webpages, be it from Microsoft, Red Hat, Canonical (Ubuntu), etc... and download a large installer/ISO, you're probably going to find yourself maxing your connection. That's the speed you are getting in real life more often than not. Or if you game, Steam and Origin tend to utilize all your bandwidth just as easily. Then if you can go back and watch video streaming services come to a crawl, and more than one, you can guarantee that data is getting throttled.
Maybe someone should come up with a VPN that masks your true Internet traffic and makes it look like speedtest.net traffic, so I actually get the 250Mb/sec that I paid for
That's essentially what just about every privacy-oriented VPN service offers. ISPs cannot tell what data is being moved about in a VPN tunnel, they just know it's VPN traffic. Because VPN traffic in general is often legitimate, they would get themselves into a tricky game if trying to throttle VPNs regardless of where the tunnel ends. There are so many VPN services, it's not like there is a VPN CDN that can be easily throttled without impacting other VPN use cases, so that's been an effective loophole when dealing with ISP throttling.
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