It's pretty actually pretty simple. The "stick of shame" as it was called in my youth is usually labelled SPD BRK. The clue is in the name... The faster you are going, the more difference they make.
Being high and clean at 210kts and sticking them out does not a lot more than make vibration.... At 320kts you can really alter your profile positively.
Most aircraft only use some panels for speed brakes. Some for roll augmentation, and all for lift dumping.