With all residential ISPs, the available bandwidth is oversubscribed, typically over 20 times. What that means is when every user gets online and starts streaming video in peak hours, the connection speeds will suffer. You will typically have higher bandwidth available between you and your ISP, but lower speed between your ISP and other providers onto the Internet, especially to very distant servers. If it is really bad, you can look at your modem signal levels, call them and complain, try to figure out where the slowdowns occur, etc.
That post is not a "proof". Web-based speed tests from different places/companies use different technologies, and they measure speed differently. You are also measuring the speed of the slowest link between you and that speed test server. Then there is a huge variation in implementation, for example some speed tests use a single traffic stream, others use 4 concurrent streams, Ookla uses up to 20 I think. Speeds even vary by browser being used, not to mention the actual variations based on time, congestion at the moment, etc.
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