Somebody hands you a small number from the back bedroom and asks what is wrong with their Wi-Fi. That number tells you the path from that phone to a server somewhere was slow while the test ran. It does not tell you which part of the path, and it does not tell you why.
Then the advice arrives: move the router, move the furniture, buy an extender. Three different remedies, chosen on the strength of one measurement that cannot tell them apart.
The Wi-Fi Client Testing Checklist runs the other direction. From the client, in order, from the radio up. See the SSID. Associate. Authenticate. Get an address. Get a gateway and DNS. Ping the gateway, then DNS, then something remote, then something remote by name. Check the MCS. Check the negotiated data rate. Then, last, run the speed test.
The order does the work. Each step sits on the one below it, so the first failure is the cause and everything after it is just consequence. And most of what arrives labeled "the Wi-Fi is broken" fails somewhere in those ping steps, where there is no Wi-Fi involved at all.
The two steps nobody runs are the useful ones. MCS and negotiated transmit rate describe the link the client actually has, in that room, right now, without generating a single byte of test traffic. A low rate says the link is poor. A healthy rate says the link is fine and your problem is somewhere upstream. That is a diagnosis rather than a symptom.
Check the band while you are there. A client on 2.4 GHz at a low rate, three meters from an access point that also has 5 GHz, does not have a coverage problem, and every coverage remedy you try will miss.
Worth knowing: a speed test changes what it measures. It saturates the link and drives up channel utilization, then reports on the network it just loaded. Run it in four rooms during family movie night and you have made things worse four times and learned almost nothing.
The speed test still has a job. Confirming what you already worked out, at the end, once.
The checklist is free, in PDF and Excel, and it has been for years.
What is the first thing you check when someone hands you a slow speed test?