This is the often spoken and often misunderstood question. In theory you should swing as hard as you can to create the maximum club head speed with many caveats. Most of us have to learn to feel like we swing at about 50% effort to make an effective swing. The average golfer swings way to hard and creates very little speed.
Why does this happen. This is most common due to lack of knowledge about how to use your body effectively. The basic explanation is that your brains first responsibility is to keep you upright and balanced. Your muscles will always default to stability before they will work to create speed. This is why most players hit a really good shot they say it feels so easy, simple and easy to repeat. The reason those swings often felt that way is they were in good balance and the muscle groups were not working to keep you stable and could create speed.
When you body works in a coordinated, balanced manner there is an absence of feeling. When your body is working hard to stay stable there is a ton of feelings because you are working hard. 
I am writing a book with two doctors on which muscles work to provide stabily and speed during the swing. Until this comes out in several months I suggest you go to the range and learn to hit balls at a slower speed. Start at 30% and work your way up 10% at a time until you notice that your accuracy and distance starts to get worse. Then work your way back until you can find what your maximum effective speed is that you can use. This will change on a daily basis and even change several times during a round.
Learn to swing at your maximum speed and watch the scores get lower. 
