I/O is cached in memory until memory is used up, then it has to wait on the disk. That accounts for the burst at the beginning. Writing to the parity array is always slower than the drives because it must first read the disk to be written and read parity, calculate the change that would be made to parity by the data to be written, write parity and write data. So, 2 reads and 2 writes when writing to the parity array.