Yup i lighter hammer spring or a stiffer valve spring would reduce gas flow.
Also but a lil tricky to acomplis depending on the gun a shorter valve knocker would reduce gas flow. The oasis gemtech is an example of an easy mod. The hammer sping is just a straight sprin removing a coil or two is an easy way to reduce fps also the valve knocker is just a str8 pin type knocker also easy to file a lil off to fine tune fps.
An m9 also uses a str8 spring easy to cut down or even put a spacer behind to reduce or increase fps. a glock however uses a funky lil hammer spring like this
Nearly imposible to adjust without a swap
What gun are u working on?