It can take a good number of shots before the piston washer conforms to the cylinder. Most people advise not changing a leather washer unless it is actually damaged.
Was it too tight when you installed it - if so it exerts too much drag. Ideally it should be loose enough to allow the piston to just fall under its own weight when the cylinder is held vertically downwards.
Also - did you lube the washer? They usually need a good soak in e.g. neatsfoot oil.
Happy Shooting!! Paul.
"We cannot solve our problems with the same thinking that we used when we created them" - Albert Einstein.