If you're on a Mac and you ever use grep, do yourself a favor and do

$ port install grep

That's likely all you need to do to install and make default GNU grep, which on my machine is a ~10x speedup over the grep that ships with Mac OS.

To wit, here are some benchmarks on my git clone of mozilla-central, with a warm disk cache.

Before:

mozilla-central$ which grep
/usr/bin/grep
mozilla-central$ time git ls-files -z | xargs -0 grep foobar > /dev/null
real 0m19.583s
user 0m18.853s
sys  0m0.722s

After:

mozilla-central$ which grep
/opt/local/bin/grep
mozilla-central$ time git ls-files -z | xargs -0 grep foobar > /dev/null
real 0m1.386s
user 0m0.754s
sys  0m0.613s

Of course in this particular case I could use git grep, which doesn't use the system's grep. It's about the same speed as GNU grep:

mozilla-central$ time git grep 'foobar' > /dev/null
real 0m1.470s
user 0m0.956s
sys  0m1.845s

/usr/bin/grep -V says that it's FreeBSD grep version 2.5.1. I don't know if the grep FreeBSD actually ships is similarly slow.