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, while the other (the fps) is the number of CPU steps that
GameMaker: Studio is actually completing in a second up to a
maximum value of the room speed itself. To get the true fps,
ie. the actual number of cpu steps per game step, use the fps_real variable.
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));
The above code will check to see if the game is in debug mode and if it is it will display the current fps on the screen.