According to many sites I found while researching this issue (for example this one), this is just normal as the bottleneck of rsync usually is CPU-power.
Results with dd and cp were near the speeds I initially had benchmarked. Seems like a 2,2Ghz dual core is just not enough for hdd-speed rsync.
During further research I also found out about this:
Correct, rsync has no option to disable the post-transfer checksum
completely. I implemented a patch to rsync 2.6.9 that adds an option
--trust-append that limits the post-transfer checksum to the added portion, not the entire file. The patch is attached. That should be
good enough, but if you really want to disable the checksum
completely, just comment out the remaining sum_update calls in match.c
and receiver.c .
Rsync always checksums the whole file which takes alot of time. Using the patch mentioned above I managed to increase rsync speed to about 90MB/s. Still not great, but much better then before. Sadly, the patch has not made it into the rsync-trunk.