Poor performance of git describe in big repos

Hi,

I'm a fairly heavy user of the magit Emacs extension for interacting with my git repos. However I've noticed there are some cases where lag is very high. By analysing strace output of emacs calling git I found two commands that where particularly problematic when interrogating the repo:

11:00 ajb@sloy/x86_64 [work.git] >time /usr/bin/git --no-pager describe --long --tags ajb-build-test-5224-10-gfa296e6

real 0m5.016s user 0m4.364s sys 0m0.444s

11:34 ajb@sloy/x86_64 [work.git] >time /usr/bin/git --no-pager describe --contains HEAD fatal: cannot describe 'fa296e61f549a1252a65a13b2f734d7afbc7e88e'

real 0m4.805s user 0m4.388s sys 0m0.400s

Running with first command with the --debug flag on gives:

11:34 ajb@sloy/x86_64 [work.git] >time /usr/bin/git --no-pager
describe --long --tags --debug
searching to describe HEAD
 lightweight       10 ajb-build-test-5224
 lightweight       41 ajb-build-test-5222
 annotated        146 vnms-2-1-36-32
 annotated        155 vnms-2-1-36-31
 annotated        174 vnms-2-1-36-30
 annotated        183 vnms-2-1-36-29
 lightweight      188 vnms-2-1-36-28
 annotated        193 vnms-2-1-36-27
 annotated        206 vnms-2-1-36-26
 annotated        215 vectastar-4-2-83-5
traversed 223 commits
more than 10 tags found; listed 10 most recent
gave up search at 2b69df72d47be8440e3ce4cee91b9b7ceaf8b77c
ajb-build-test-5224-10-gfa296e6

real 0m4.817s user 0m4.320s sys 0m0.464s

Which has only traversed 223 before coming to a decision. This seems like a very low number of commits given the time it's spent doing this.

One factor might be the size of my repo (.git is around 2.4G). Could this just be due to computational cost of searching through large packs to walk the commit chain? Is there any way to make this easier for git to do?