Bernd Schubert wrote:

Try to increase the read-ahead size of your lvm devices:

blockdev --setra 8192 /dev/raid10/space

or increase it at least to the same size as of your raid (blockdev
--getra /dev/mdX).

This did the trick, although I am still lagging behind the raw md device 
by about 3 - 4%. Thanks for pointing this out!