"Speedcheck.org" uses Ookla in the background and whatever server/connection they are using is trash. "wifiman.com" seems to have a legit backbone. (granted I have quite a few file sharing(s) going at the moment) As far as Testmy.net goes, with a 20mbit upload your test should be fine.. Connections over a couple hundred mbit struggle here, on this current revision. @CA3LE will be releasing a new revision of the current Testmy.net testing methodology soon to handle excessive upload speeds, as well as some new tweaks.
You have to remember, there are alot of factors when it comes to internet speeds. Location, route, congestion, etc. Have you tried changing your server & testing on others? https://testmy.net/mirror
Also, this may be worth checking out if you hadn't already;
Another good one to check out is here: https://www.waveform.com/tools/bufferbloat
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