A while back, I was shooting some owls in flight using a D70 and a 70-300 AF lens. The first day I had limited success, the camera would not focus quick enough. So I had about 10% keepers. As I was leaving, I started talking to someone with a Canon 1D Mark 2 and asked him for some hints. He suggested that on lower end cameras such as the D70, there is a single processor that sequencally takes care of all aspects of the camera operation such as focus, exposure, white balance etc where as on higher end cameras there are more than one processor operating in parallel. So the next day I set everyting to manual, the exposure, the white balance but still used the autofocus. Lo and behold, the focus was much quicker and my keepers went to 70%.
At any rate, I still find the focus on the D70 slow, but does anyone know if there are multiple processors on the D200 which will most likely be my next camera?
i can assure you that the manual exposure doesn't help the autofocus system. A change in lighting or subject could explain the difference in AF speed for you.