An ESP8266 running NodeMCU can yield maximum frequency of 1.44 kHz with a duty cycle of 61%.  The wave form is given below.  The code used to perform this test is:

pin = 8
gpio.mode(pin, gpio.OUTPUT);
while true do
 gpio.write(pin, gpio.LOW)
 gpio.write(pin, gpio.HIGH)

Without clearing the watch dog (tmr.wdclr), the device can yield a frequency of 1.84 kHz.  Without clearing the watch dog, the device reboots after a few seconds.

ESP8266 Wave Form

Click on the picture to enlarge it.

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