Speed Tests
Speed tests are a specific use case, so take results with a grain of salt and if you're concerned that you're not getting the services you pay for, try multiple tests and make sure you understand what's actually happening in each test, as follows:
Speed tests are typically an Internet Browser based test that check the TCP capacity between that device and typically remote test server.
Flaws:
Tests have to compete with other services, so if you are gaming or watching netflix while you speed tests, your tests will be lower because it has to share
Tests are often done through a consumer router and often over WiFi. These two things can be substantial limitations.
Speed tests servers could be very near or very far. If you test from Kansas to a server in Florida, you're putting a lot of 'internet' between you and the server. On the other hand if you test to something hosted by your ISP then you may not be getting an honest result, it's not actually a test from the internet.
That said, they are still valid tests especially if you are aware of the issues and you do multiple sites.
Other types of tests exist, such as tests from your modern router. Eero for example does a UDP test with a smaller MTU so it often shows a very optimistic number.
Buffer Bloat speed tests:
https://www.waveform.com/tools/bufferbloat
https://fast.com/ (check more info)
https://www.speedtest.net/ (detailed results)
Other Speed Tests:
https://openspeedtest.com/
http://www.dslreports.com/speedtest