Abysmal sparse file performance!

[grarpamp]

I have a 5.5GB file, mostly sparse. Tar performs far[!] better than rsync.
I have no ideas yet, so just an FYI as to current state.
FreeBSD 8.1 i386 zfs
Yes, I know the blocks used differs but don't know why yet, could
be just how zfs does things or related to the large amount of sparseness.
There are no media errors, CPU/IO load or anything like that and the source
and dest paths are on the same filesystem.
I've not tested times for files that are, say, 90% full instead of 90% sparse
Though a 50% 35MB file was 8.5x slower than tar and had identical
block counts and sha256 with both.

/usr/bin/time rsync -HaxiS ./a ../
>f+++++++++ a
      271.13 real       101.44 user        95.62 sys
l -s ./a ../a ; rm -f ../a
blocks bytes
3625 5535932416 ./a
3769 5535932416 ../a


/usr/bin/time tar -cf - ./a | /usr/bin/time tar -C .. -Sxf -
       57.67 real         1.10 user        27.77 sys
       57.67 real        10.68 user         5.87 sys
l -s ./a ../a ; rm -f ../a
blocks bytes
3625 5535932416 ./a
2977 5535932416 ../a