How does the Internet Speed Test works?

The article is a good high-level overview of how speed tests work but there are a lot of technical details that vary greatly from one carrier to another and between speed test sites.

- some speed test sites run multiple parallel streams (TCP, FTP, HTTP) then show the aggregate. This tends to show an exaggerated result since most of us are not running a lot of parallel streams in normal use, although it is not uncommon to have between 1 - 8, that is a more realistic number (some are 50 or more, not realistic).

- some carriers detect known speed test server 5 tuples and give those priority over normal user traffic (and yes, I know this because I have worked for some who do that, in fact most do). Speed test that rotate the 5 tuple overcome that issue. you can also be test this yourself using a VM of your own on Azure or AWS.

- depending on the carrier, some have other types of throttling they do for different types of traffic that are not applied to speed test traffic. Some will throttle video for example, or if it is a wireless carrier they may throttle if you are tethered rather than directly on their device, or may have different priority for your plan.

there are a lot other factors such as your end user equipment, if your CPU is fast etc. also, how you are connected, wifi vs. wired.

Speed tests are a very general measure of the performance you may get in normal use from your provider, but not perfectly indicative.

Everyone deserves the right internet speed and everyone wants the best bang for a buck. To ensure our internet bandwidth, we all run speed tests from our Internet Speed Provider or public speed test tools like fast.com or speed.cloudflare.com and more. But do we know how the speed got measured under the hood?