room_speed is the target steps per second of the current room. It is not the actual steps per second as if you have a slower computer, it will actually be less. To find out how many steps are actually executed, use fps.
By default room_speed is set to 30. A more common room_speed is 60. This is because 60 frames per second looks alot smoother than 30 frames per second.
You can change the room_speed before start up by going to the Room Settings Panel / Settings Tab. There you will find the current room speed of a particular room.
Example of Use
room_speed = 60;