OpenSceneGraph performance between Qt 4.7 and 4.8

I am working with OpenSceneGraph 3.0.1 (latest stable release) and Qt. OSG has a module named osgQt for creating an OSG rendering widget. I noticed a strange drop in frame rate from Qt 4.7 to 4.8. Since 4.7.3 the frame rate has been maximum 60 frames per second on each computer. With 4.8, the frame is maximum 38-40 frames per second on each computer. Definitely too many to agree. I do not understand why! Is this a bug or what?