Keep in mind the “crocodile effect” (big mouth no ears…)
The WiFI AP might loud and clear but can it hear weak signals from smartphones, tablets and iot devices?
There is a constant back and forth between the network devices. For example, loading a web page is a big stack of smaller pieces of data each of them has to be confirmed by the recipient. The WiFi AP will keep repeating the last package before sending a new one until it has received confirmation.
When it has trouble hearing the weak signal it will significantly slow down the user experience.
Scan the WiFi spectrum close the WiFi AP to make sure there isn’t much interference to increase the Signal To Noise radio (SN) of the weak devices.
Running SpeedTest is an indicator but please keep in mind that these “test” are kind of rigged and do not represent the actual user experience in browsing the internet, streaming content etc
It’s often better to have multiple WiFi AP’s with low power than one big one. This is where in the consumer world the “Mesh” devices have an big advantage assuming they are setup right.