The problem is that you are hindering Premiere in decoding by using a Quicktime container-based format. When you shoot .MOV (whether to ProRes or DNxHD), Premiere has to go through the Quicktime engine in order to decode, limiting itself to 32-bit operations. When you shoot to the nanoFlash (XDCAM .MXF) Premiere can use its own internal engine to decode your footage with full 64-bit operations.