If you have a Xfinity box for internet or might apply to all is First unplug the power for a 10 seconds and wait a while after it restarts as it could be updating. ( They send emails out when to also do this after working on or doing some update).
X has speed tests in its apps. It appears to be a speedtest.com with a different logo. X speed apps will show the speed to you box.
https://speedtest.xfinity.com/While spending a day with them here at the house trying to figure out my speed issues we came up with this.
For Windows, mac should have something also.
Open Task Manager, Select Performance and you should see CPU, Memory, HD and Ethernet. 1 click on Ethernet with make it the bigger window.
Bring up speedtest and resize the window so both speed test and Task manager are visible.
Start speed test and watch Task manager.
1st of all I have never got the numbers to match but they are close.
And then I can make to argument that they are different.
I just ran the above link and only got 498 up Out of 1 gig
https://www.speedtest.net/ 385 d 2.28 up (1 Gig plus and 25up is my package)
Control panel concurs

And I can different readings with my phone and the TV.
Alan