On Wed, Dec 23 2015, Marc Smith wrote:

Solved... appears it was the write-intent bitmap that caused the
performance issues. I discovered if I left the test running longer
than 60 seconds, the performance would eventually climb to where I'd
expect it. I ran 'mdadm --grow --bitmap=none /dev/md0' and now random
write performance is high/good/stable right off the bat.

Keeping a write-intent bitmap really is a good idea.
Using a larger bitmap chunk size can reduce the performance penalty and
preserve much of the value.  It is easy enough to experiment with
different sizes.

NeilBrown