Centaurus :
During an RTO positioning the thrust levers to idle thrust and raising the reverser levers to lock will not raise the speedbrakes.
When the sleeves are fully opened ,then you may select reverse thrust, at which time the speedbrakes will deploy.
You may be right, it may not be a question of N1, I am not sure about that.
But it is a delay from the moment you raise the reversers untill you are able to raise the reverser lever to command reverse thrust., about 2 sec, I guess.