Speeding up a loop (assembly)?

Cool, I didn't know it was that simple (and that spi.send can return arrays). But if I understand correctly, there is no way to shovel the data from SPI directly into the buffer of the waveform (waveforms bring their own buffers), so I always have to copy it? And I also have to temporarily allocate the space for the outgoing zeros, so effectively I need 3 times as much memory as the received payload?