[Azuki]

Having a harder floating valve spring will DECREASE the FPS. The Spring in the floating valve (at least in WA, and I believe in TMs too) pushes the floating valve to close it after the BB is shot. The only spring that should increase FPS as it becomes stiffer is the hammer spring.

Hammer Spring Harder = Higher volume of gas released
Recoil Spring Harder = no effect on FPS, more gas required to cock
FV Spring Harder = Potentially faster closing of valve, less FPS, more blowback pressure
Magazine Valve Spring harder = Less Volume of gas released

and vice versa.