I just had a thought that turns out to be useless at the moment.
If there were a user-selectable usermod that you could simply enable or disable that would turn the strip into a 0-120 Hz display that showed current frame rate by turning on a percentage of led’s, you could see how fast or slow it was - even if wifi were not connected (in AP mode).
That would give you the theoretical maximum update rate for your LEDs and WLED setup.
Then you could enable wifi and see how much of an impact it has.
If E1.31 is controlling, then the frame rate is depending on xLights update rate, wifi 2.4GHz bandwidth and signal strength, # of LEDs, ESP8266 or ESP32, and which version controller is used (40MHz or some other clock speed?). Probably a few more things, but those are things you can change and then do the same exact % frame rate test.
Again, I wish this was available, but as far as I know, it is not.
It would not be difficult. Savvy usermod developers could easily force the percent effect, read the frame rate, and then set the % illumination. Heck, could even send it via MQTT automatically so you could see if frame rate is changing linearly with # of defined LEDs.
If you take it off your network, putting it into AP mode, and connect to it, what’s the frame rate then?
If it goes way up, you have some nosey or noisy wifi devices interfering with WLED.
If you are using xLights or jinx or other external protocol driving the led’s, try turning it off and see what the fps becomes.