Speeding up a loop (assembly)?

You could also include any required SPI commands at the beginning of the sendData array, and could then use ArrayBufferViews to cut out the data you need without a copy:

var sendData = new Uint8Array(4+256);
sendData.fill(0);
sendData[0] = ...;
sendData[1] = ...;
sendData[2] = ...;
sendData[3] = ...;
// now in the loop:
var result = SPI2.send(sendData);
var data = new Uint8Array(sendData.buffer,4,256);