You're not doing "100b" (byte? blocks? what was your intention?) chunks, you're doing 512 byte chunks. That is always slow because the RAID is using 512k chunks. That means that for every 512 bytes the RAID system has to read a 512k chunk, update 512 bytes in that chunk, compute its parity, and write the data + parity out to disk. It has to do the update, parity calculation and 512k + 512k parity write every time for each of the 1000 blocks you're writing. The read is only done once as that's kept in the buffer cache, and the entire operation only takes one RAID chunk.
So, yes, that's normal. Don't do that :-)