To clarify things even further. The input voltage has a resistor between it and the metal sensor surface. The size of the resistor has a tradeoff between sensitivity and speed. A tutorial written by the makers of Teensy recommend 100K to 50M. https://www.pjrc.com/teensy/td_libs_CapacitiveSensor.html So the 1m resistor recommended by Jeremiah Rose is probably a good value.
Speed is important for a musical instrument but equally important is not missing a note.
A few more design considerations that could mitigate the loss of sensitivity. I think if other aspects of the sensor are designed better then you can probably more reliably use those lower/faster resistor values.
1.) sensitivity goes up when you increase the size of the sensor. I don't know how far that scales but it works within reason.
2.) If this is a pcb touch sensor. The sensor should be solid but for copper ground plane it is recommended to have grid fill pattern instead of solid. Ideally at 45 degree angle to the sensor surface. If sensor goes up and down use 45 fill. This is probably something to do with emf.
3.) Sensitivity goes down when there is no good channel to earth. So basically if you can connect ground to metal that the user even occasionally touches that will help. The problem builds up over time. 10-15 seconds in one user's case. If using usb power to a large object like a laptop then it is not an issue, even when the laptop is not plugged in.
4.) When putting any cover on the sensor you hope it to be stuck to the sensor. An air gap causes inconsistency. https://touchinternational.com/touchscreen-adhesive/ They say use 3M 4956 mm. 3M 467MP seems similar and cheaper.
5.) If the user has very dry hands the sensor might not work well. Something to consider when testing. Lotion is an option.
6.) Teensy recommends a 1k resistor on the return signal pin to help avoid static electricity damage.
7.) Other metal too close to the sensor could be an issue. I think .2mm was the minimum. Think about mounting holes. Screws.
8.) Capacitive sensors need calibration. Wires moving around inside is an issue and after opening it up and closing it you may need to re-calibrate.
Hopefully this will help any future musicians looking to blend creativity and technology. It has become a relatively approachable project to beginners.