Several years ago, I stumbled across Google's Internet speed test. That prompted me to look at several other Internet speed testing tools. The post is here.
This article on CNET prompted me to look again. CNET had a couple of tools I hadn't heard of before so I ran them against my previous set of tools.
At my house I have a 200Mbps Xfinity connection. I was using my ThinkPad X390 with an Intel(R) Wireless-AC 9560 160MHz Wi-Fi adapter. Intel says that adapter can deliver 1.73Gbps so that probably wasn't a limiting factor.
|* were in my earlier test|
Conclusion: Mox nix!
The results were much more sensitive to other traffic than the accuracy of the various tests. In my initial tests of speedof.me and testmy.net, they were both around 125Mbps. I retested them and they both came in over 155Mbps. A third test gave the above results.
A more extreme demonstration of interference was at my daughter's house who has a 1Gbps Xfinity connection.
I didn't have the opportunity to rerun the tests at this location. In hindsight, there were streaming applications running outside of my control during the testing.