Interesting experience. Your brain injury could have cause an area that competes with the area that plays the piano. If this area is 'dormant,' the other area competing for brain resources can access those unused resources. It is the same reason why blind people have excellent hearing and touch.

The sluggishness can be a slowed processing speed. My cognitive processing speed and some others are 25% of normal. It shows up on finger tapping tests. You don't notice the sluggishness unless you have a time standard to compare it to. It is likely even more obvious to an observer.

I have a similar odd situation. If I try to do brain training exercises, I do better if I stop trying and just let the answers/reactions flow. It is kind of scary as if my mind is doing the exercise without me.

My best to you.