A common enough tip given is that you should swing your driver at 80% effort. Elite golfers commonly say they swing their driver at this 80% effort level.
How true is this?
Last year (2012) on the PGA Tour the average swing speed with a driver was 112.64 mph as measured by TrackMan launch monitors at Tour events. The highest average swing speed was JB Holmes with an average speed of 125.12 mph. Bubba Watson's fastest recorded swing was 127.02 mph.
If the average PGA Tour player was swinging at 80% then their maximum speed should be 140.80 mph. This isn't the case. From my own observations on TrackMan when asking golfers to hit it as hard as they can, they rarely gain no more than 5mph speed from their 'normal' swing.
This is a good example of 'feel' versus 'real'. Golfers feel they are swinging with less effort and feel it is significantly less effort then their maximum but the reality is they are swinging only marginally less then their maximum.
A golfer with a maximum distance of 250 yards will only hit it 200 yards with an 80% swing. Who wants to hit it 50 yards less? It's very common to hear: 'I need to slow my swing down', my answer to this is 'You want to hit it shorter?'. Slowing the swing down means shorter distance.
Don't slow down, smoothen the swing out, take 1-2% off (it may feel like much more), sequence might improve, energy might transfer better, possibly leading to more speed and strike might be more centered (definitely leading to more ball speed and longer distance).
If you want to hit it further, don't slow your swing down. A swing is never too fast (unless the ball has gone too far). It can be too quick though, too quick = poor sequence. So next time don't say you were too fast, say you were too quick.