sudo sysctl -w net.inet.tcp.delayed_ack=0
After that, my SMB transfers jumped to 1 MiB per second, minimum. You can presumably make this change permanent by adding "net.inet.tcp.delayed_ack=0" to the file /etc/sysctl.conf. This will then be applied during system startup.
Update: Here is a nice explanation why the performance of OS X suffers sometimes, when using delayed ACK, and also why it might be useful in some cases.