Because you are are using Kingston SA400, which are some of the slowest SSDs available, not all SSDs can sustain 500MB/s writes, if fact most can't, unfortunately I missed that your array also won't be very fast since there's a 1TB disk, but pretty sure you are seeing low speeds due to device limits, that's why it starts reasonably fast, while it's being cached to RAM then it slows down when it's limited by the device write speed.