slow transfer speeds between datasets

[digity]

I'm in the middle of migrating TBs of data between pools and datasets. In the target pool (4 x 4 x 2 TB RAIDZ1) there's non-dataset data (sitting at the root of the pool) that I'm trying to move into a dataset with the mv command and I'm getting 16 to 32 MB/s write. But when I'm migrating datasets between the source and target pools with the zfs send/recv/rollback method I get 320 to 480 MB/s write. Why is the data transfer within the same pool so slow? How can I get faster transfer speeds within the same pool?