However, that level of control just isn’t possible in the context of the Arduino’s ESP8266WebServer library. So what’s to be done? The next best thing turns out to be a simple delay(1 ...

If delay() has a low value (for example 50ms or 0.05s) the fading speed will be higher because the Arduino will keep the voltage at pin 11 at a certain value only for 50ms then executes the next for() ...