How to make fast (low latency) capacitive touch buttons
It seems that you have found low power products that do not need to be so fast as they have not been designed to be used on high performance musical instrument needing fast response time, but as simple user interface buttons that need to be reliable (think of stoves and ovens that need to be robust and extremely sure not to trigger easily by  accident or house can burn down).
What you can do is to find a reasonably powerful microcontroller with hardware support for capacitive sensing, and fine tune the accompanying sensing software library to fast response time. Most likely your device would have a microcontroller anyway, or at least it would benefit from it.