Take its results with a pinch of salt if you're on a very fast connection, since download tests will be limited by the performance of the phone chipsets and of your Wifi specifics, but on slower connections that you're worried about, note that it works very well indeed.
From the Store description:
Use Speedtest for easy, one-click connection testing in under 30 seconds—accurate everywhere thanks to our global network. Millions of people each day use the Speedtest website and mobile apps to test their internet speed. Now, the most accurate and convenient way to test your speed lives on your Windows desktop.
- Get your ping, download, and upload speeds within seconds
- Troubleshoot or verify the speed you were promised
- Track prior tests with detailed reporting
- Easily share your results
Back in 2017 I said:
It all works as expected, nice and simple and just one button to hit. Results are, as I say, limited to device capability if your Wifi is insanely fast. My (cough) 5GHz Wifi on a 160Mbps fibre connection was far too much to be line limited - you're more likely to use Speedtest when your Internet bandwidth is marginal and you're trying to find ways to improve it. The most I could get a Snapdragon 810-powered Lumia 950 XL to achieve on the download side was 93Mbps, while my 820-powered Axon 7 (running Android) showed 110Mbps, etc.
Which is curious, since I'm now getting twice that throughput in 2020, on the same 950 XL. The hub/router is newer, so perhaps improvements in Wifi are showing here and also that the 950 XL wasn't quite the limiting factor I thought it was!