I wouldn't feel there is a problem with the springs. The only reason to replace them is if they are weak or out of spec. The adjustment does this. The valves are designed to open a specific amout so the air in the chamber can enter and exit. The exact amount of air can effect engine performance. So if the valves are out of adjustment you will get that tap tap noise. Louder than normal ofcourse but it actually is alowing less air to enter and leave the combustion chamber therefore affecting the performance.