Arduino ESP8266 direct/fast control of the digital pins
After a weekend of searching and trying, here is what I was looking for:

It is slightly different than for the AVR chips. There is one register for setting the pins to HIGH and another one to set the pins to low. 

Setting the pin HIGH:

GPOS = (1 &lt;&lt; PIN_OUT);


Setting the pin LOW:

GPOC = (1 &lt;&lt; PIN_OUT);


using this, I managed to speed up my code by a factor of three. It is not as dramatic as for the AVR chips, since the digitalWrite function is much more simple for the ESP8266.