The slow response on the sticks is a problem with the POT on the Joystick. Some have said they have been successful cleaning the contacts with Oil-less electronics spray cleaner. With the ST-16 powered off, spray some into the openings on the side of the joystick and moving it around thoroughly. If that works, great.
The other issue is not dirty contacts, but the POT becoming loose which make the contacts movable when moving the joysticks so they do not respond appropriately. The POTs are held to the joystick assembly by two very fine screws. After use the POT wears and becomes loose and begins moving (Very Minutely), enough that you can barely see the movement, but enough to effect the POT performance. Some have replaced this POT, only to have to replace it again in the future when that one wears down.
A very simple and inexpensive fix it to immobilize the POT by securing it with a drop of adhesive, Hot Glue being very effective. Below is a video to show how to open the ST-16 and another video showing how and where to place a drop of hot glue to permanently fix this issue. Neither are my videos, I found them when researching the problem, there are others, but these are well explained and work.
If you use these fixes, please give them a like or a Kudo for their info. I have no connection to them, but they have provided fixes for others that could save money, time, and prevent an UAV. So I feel they should be recognized for that.
Taking ST-16 Apart Video
Hot Glue Fix for Faulty Joystick POT
My suggestion is, while you have the ST-16 open and as a preventative measure, place a drop on all of the POTs to prevent any of them from working loose in the future. This could prevent future costs in replacing the POTs, downtime to fix them yourself, and/or loss of your bird should they go out while in flight.
Happy and Safe Flying to All!