To me, the difference between fast and slow is knowing the trigger break point subconsciously. If you dry fire enough to know the exact break point and can "will" it to break when the sights cross the target, it will be a bullseye everytime....so how to???
First dry fire a bunch of times, first learning the break point without aiming the gun. Do this both fast and slow, but focus on a certain type of squeeze. I put my finger print on most triggers and first joint on DA's. I squeeze with an increecing force squeeze. I get to where I can do this fast and slow with minimal gun movement. Then I bring the sights into play and focus on breaking as I approach the POA.
Next, work on grip and stance until the gun recoils straight back. Then add grip pressure until you reduce the recoil distance. Straight back is seen as the front sight rising up slightly and the you push forward while squeezing the trigger to break it as the sights align with the bullseye. Too slow and you hit low or feel a sight bounce from stance lock. Too fast and you hit high.
Then bring it all together with doubles where you aim the first and push the second back to the same spot. With practice, the shots can be within a couple inches of eachother.