Why is "git tag --contains" so slow?

["Theodore Ts'o" <tytso@mit.edu>]

I'm trying to write a script that can determine the first kernel release
(i.e., a tag of that matchs v2.6.*) that contains a particular commit.

I can do this using "git tag --contains <commit-id>", but it's quite
slow.  It takes something like 8-9 seconds.  (8.5 seconds of user time,
8.6 times of wall clock, to be precise).

I can get the information much faster using "gitk -1 <commit-id>", which
finishes painting the screen in under 2 seconds, but that throws up a
GUI and then a human has to pull the information out using their eyes.
(Yeah, or I could figure out where in the 11,631 lines of Tcl script the
"preceeds" line is calculated, but I figured I'd ask here first.)

Is there a better way of calculating what I want from the command line
using the built-in native git tools?  And if so, why is git tag
--contains apparently 4 times slower than gitk at performing this task?

Thanks,

						- Ted