In GameMaker: Studio there are two main ways that can be
used to tell the speed at which your game runs. The room_speed (as specified in
the room editor) and the fps (frames per second). These
values are often confused, but basically one is the number of game
steps that GameMaker: Studio is supposed to be completing in
a second (room speed), while the other is the number of CPU steps
that GameMaker: Studio is actually completing in a second (the real
fps), and this value is generally much higher than the room speed,
but will drop as your game gets more complex and uses more
processing power to maintain the set room speed.
This read-only variable returns the current fps as an integer value. Please note that the function will only update once every step of your game and so may appear to "jump" from one value to another, but this is quite normal.
draw_text(32, 32, "FPS = " + string(fps_real));
The above code will check to see if the game is in debug mode and if it is it will display the current real fps on the screen.