zfs code - May 2010 - ZFS is really really slow with fchmod...

Using build 139, I''m seeing some very strange performance with pax to 
copy files from one ZFS filesystem to another. The truss output is as 
follows:
...
  0.0796 read(4, "7F E L F0201010601\0\0\0".., 131072)   = 131072
  0.0799 write(6, "7F E L F0201010601\0\0\0".., 131072)  = 131072
  0.0848 read(4, "AB *7F E \CD [ h q "11E2".., 24120)    = 24120
  0.0850 write(6, "AB *7F E \CD [ h q "11E2".., 24120)   = 24120
  8.7385 fchmod(6, 0755)                                 = 0
  8.7387 fchownat(5, "libsun_sas.so.1", 118747, 10,
AT_SYMLINK_NOFOLLOW) = 0
  8.7388 fchmod(6, 0755)                                 = 0
...

Can anyone suggest some things to look at with dtrace?

Darren