Here is how I diagnosed mine step by step:
1. Run a sustained 5-minute speed test, not a 3-second one.
2. Check the actual measured throughput, not the peak burst.
3. Switch from Wi-Fi to Ethernet and test again.
4. If Ethernet is fine but Wi-Fi is not, you have found the cause.
In my case, Wi-Fi showed 150 Mbps peak but only 18 Mbps sustained during a 5-minute test. Not enough for 4K.