fps - actual number of frames drawn per second
fps contains the actual number of frames drawn per second, unlike room_speed which technically contains the maximum number of frames to be drawn per second (whether it actually does or not is dependent on the speed of the computer running your game).
Example of Use
room_caption = string(fps);
This will display the frames per second in the room_caption.
room_caption = string(room_speed - fps);
This will show the lag in the room_caption.