we've investigated that issue and found our that IPP function called in OpenCV has not enough optimization (one with Ipp32s data type). We would recommend to use IPP function which support Ipp32f data type instead. It is optimized in IPP 5.3 and provides about 50% speedup over OpenCV without IPP.
Please check OpenCV sorce forge project for the latest update or contact with Vadim Pisarevsky directly on that.
From our side we will work on providing additional optimization in IPP function wich work with Ipp32s data type (in general it provides more detection accuracy)