Other than a light polish of the sear/ trigger engagement surfaces there isn't much you can do. Don't fit a lighter trigger spring as this won't work......well it will lighten the trigger pull bit it will also reduce your muzzle velocity.!!! ..Oh yes it will.
This pistol has a " pull open " type of valve which requires a sharp short pull to release the compessed air behind the pellet.... if you pull the trigger slowly ( i.e. with a lighter spring) the air will escape more slowly from the valve reducing muzzle velocity.
It is rather like using a pea shooter......a slow blow will just about shoot the pea out, but a sharp blow will send it across the room.....see.
Pete.