Optimizing specific code

Hi,
I'm searching of an optimization of the following code:

void prepareData(uint16_t* dataOut, uint16_t* dataIn, uint32_t length)
{
        uint32_t i;
        for (i = 0; i < length; i += 2)
        {
                dataOut[i] = (dataIn[i+1] >> 4) & 0x03FF;
                dataOut[i+1] = (dataIn[i] >> 4) & 0x03FF;
        }
}

It's just swapping 2 16-bit words. shifting them by 4 and setting the upper 6 bits to 0.
I already tried the hints from http://www.keil.com/support/man/docs/armcc/armcc_cjajacch.htm . But its getting slower with decrementing counter.

It's taking about 50ms (55ms with decrementing counter) for a length of 350000.
Target: AT91SAM9260, executed from external RAM.