I don't think it's the Software-Raid Level which decreases the speed.
I "degraded" the drbd and tested the speed again:
root@glowstone /dev # dd if=/dev/zero of=/dev/storage/ovz-150 bs=5G count=1 oflag=direct
0+1 records in
0+1 records out
2147479552 bytes (2.1 GB) copied, 10.0859 s, 213 MB/s...