Why is DRBD write performance so low?

While setting up a new system I noticed that write performance is very low, only about 35MB/s. This is with and DRBD 8.3.1. The disk device is a md device (software raid) and when I run the following test: dd if=/dev/zero of=/dev/md4 bs=64k count=100000 it will give me approx. 160 MB/s. If I do this against /dev/drbd0 the maximum I get is 70 MB/s when I disable DRBD on the secondary. The 70 MB/s with the secodary enabled I do manage also when I play with sndbuf-size, max-buffers, unplug-watermark and al-extents. But why is it limited at 70 MB/s when secondary is disabled? The drbd.conf looks as follows:

Interesting post on DRBD performance.

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