Modify-Tech explains the issue of low FPS in AEGs and one cause is can also be the hop-up spring. They present their Tapered Hop-Up Spring in their XTC as a good solution... "There are lots of variables causing AEGs low FPS issue. The air seal is a key point. The hop up spring, though small, accomplishes big as it can pushes the hop up chamber back into the air nozzle to reach a good air seal.