Well if the only thing that changed is the destination "speed test" server on the internet, then the cause could be the TCP protocol itself. I'm referring to TCP flags like the MTU etc. These will have different settings on the router Wi-Fi and Ethernet interfaces, and each speed test server will also have its own settings which may or may not be the same... I would start by tweaking the Wi-Fi MTU, try lowering it a small amount (I use a value below 1500 due to PPPoE overhead, I vaguely recall it was 1492 or 1472).
For example: If you're using a MTU of 1500 and the first speed test server is using a MTU of 1492, then every single packet you send to it will be too large by 8 bytes, so it will be split into two packets which is called "packet fragmentation" (potentially halving throughput, causing strange delays and other unusual behavior). If however the second speed test server has an MTU of 1500 just like your router does, then there would be no fragmentation and you should achieve much better speeds (assuming all other conditions such as latency are good and no use of PPPoE). If you reverse this example (so you have 1492 MTU and server has 1500 MTU), then there is no problem as your packets are 8 bytes smaller than the 1500 Maximum Transmissible Unit - A.K.A the MTU...
I don't know for sure this is the cause of the issue you experienced, but it does fit the symptoms you described. And I'm not a TCP expert, so perhaps others could jump in with recommended MTU values (with and without the use of PPPoE) and point out any errors I made above, as it's been a long time since I last dealt with the MTU issue.
