Leaving it cocked won't do much if anything to reduce fps. It's very unlikely that the spring is taken past its elastic limit in use. Slowing down the piston by wrapping a little PETE tape under the seal to increase friction between the seal and cylinder is quick and dirty but likely to change with time. It really needs to be a permanent fix. Shorter or less powerful spring, or add a couple of layers of mouse mat to the front of the piston to shorten the stroke and hence cylinder air volume perhaps?
