A reply on the DAZ forums indicated the problem is with the code controlling updates to the "Loading Progress" dialogue. It's dedicating too many clock cycles to the progress bar updating, begins to thrash in memory, and forgets to hand back to the "load the next bit of .cr2 file" routine. And it is that specific: loading files already in D|S format seems to be unaffected, as BColeman said above. If the D|S window is minimised, no progress bar is displayed, so no wasted time and that's why the load is almost back to normal.