Kit Westneat commented:

"Older versions of tar have to read in the entire file to figure out
what parts are sparse. Newer versions should skip that if the # of
blocks are 0, but I'm not sure if that made it into lustre-tar yet.

Here's the patch:
http://lists.gnu.org/archive/html/bug-tar/2010-08/msg00043.html
"

This hasn't made it into lustre-tar yet, and may be worth looking into.