With gas blowback systems, you want a good balance with your recoil spring. Too stiff, and your gun won't cycle all the way, and may not load a round into the chamber (like my WA M4 does with the heavy recoil spring in it in cold weather), or even worse could not push the hammer back at all and just vent all the gas in your mag out the front end. Too light, and it wont allow much gas into the system to propel the bb downrange before letting the slide or bolt snap backward.
If you're going for faster and harder cycling, you would need to combine a thicker/stiffer spring with a high flow valve on your magazine to allow enough gas through the system to properly cycle the slide or bolt. This will wear on the gun a little faster, as well as up your muzzle velocity, so you may not want to do it if you plan on keeping it a light maintenance gun, or if you plan on using it somewhere like Vanguard.
