jb847

What are your speed measured here on TestMy?

Nice downloads...up isn't so good but that is cable for you. 
Frontier (previously Verizon Fios) Fios Fiber 50/50 speeds...love the upload for work related needs.

CA3LE. And you should also mention that most of the OOKLA based test sites' speed tests won't run on an iOS device because the tests require Flash (the memory and battery hog technology that's not allowed on iOS devices). So kudos for you having iOS compatible testing. And although Speedtest.org now supports iOS/Safari, the results are way off. They even have a disclaimer:


"Warning
For best testing experience and accuracy, Broadband Speed Checker requires at least version 8 of Flash. Please update your client by clicking here."

And when you try the support Contact Us on Speedtest.org, you get:
"Not Found. The requested URL /contact/ was not found on this server."

Plus you can't choose a server like you can on TestMy.

There is an OOKLA Speedtest app for iOS that allows you to pick servers, and the results trend kind of like the results I get here on TestMy, but the UI and the results displays and overall functionality of the app is orders of magnitude less than this site.

For iOS device users, nobody else that I've found is even in the same universe as TestMy.

I'm with Centurylink, I pay for 1.5/256 service, my downstream rate is 1760Kbps and I always get 1.9 Mbps or better on this website. Speedtest.net is usually right around 1.5 or slightly less. Its hard to believe I receive more than I am able to get..

I know why Testmy.net is 10x better.
Ookla Sucks, TMN doesn't
It's just plain simple

If this advice was given for a mobile Internet connection, what they likely mean is that running speed tests gobbles up the available data allowance.
 
For example, many of the popular mobile phone packages in Ireland and the UK have a 1GB data allowance, such as the following example I picked from the UK O2 website:
 

Running a speed test on 4G typically uses up to 100MB per direction per test depending on the 4G speed.  For example, if one is getting around 50Mbps, TestMy will usually download about 60MB in total.  This is double for the uplink direction as TestMy first downloads each block size to run the upload test.  So for a typical 20Mbps uplink, it will use around 40MB in total.  For faster 4G areas such as 20MHz areas or LTE Advanced (or 4G+), these figures double or triple.  Speedtest.net uses a similar amount of data, around 100MB total for a 4G connection delivering about 50Mbps down and 20Mbps up and I'm sure it's much the same with other speed test providers. 
 
So assuming a speed test uses about 100MB per test (combined up & down) and the user has a 1GB monthly allowance like the above tariff, that person just needs to run the test 10 times to use up their data allowance without doing anything else online. 
 
Basically, for anyone with a 2GB or lower monthly data allowance on their mobile handset, run the speed tests sparingly on 3G and avoid running speed tests if at all possible on 4G, apart from on the last day of the billing cycle and you have at least 200MB left (500MB for an LTE Advanced / 4G+ area).