Why do hammers and keys sometimes return slowly ?
Several possible reasons, but usually because of corrosion on the pins of the flange joints in the action.
The piano action has lots of hinges that pivot. In these pictures, you can see the greenish oxidation of the copper in the alloy that the pin is made of. How to fix it? I use a lubricant called CLP that is great for dissolving this junk. Some pins are so corroded, however that there is no other solution but to replace the pins. This extra friction can be a real problem in older pianos. A pin joint in a piano action part should have 2-4g of friction resistance. A badly corroded joint can have 20g or more of resistance. Whenever we have a piano action in for repairs, we always address excess friction in action parts.