The 18-200mm has a fast focusing mechanism, which results in fast AF in good outdoor light. However, the max aperture is already f5.6 at 70mm and eventually f6.3, so it can't collect enough light for fast AF. I get a similar problem with the 16-50mm Powerzoom at 50mm. It is quite slow to focus in indoor low-medium light. The 45mm f1.8 though is still quite fast in the same light. I guess the situation would be similar with the 50-150S at longer focal lengths.