If on a git repo, use `git grep`. Similar options, much, much faster.
Example, on a directory with 11K+ files:
grep -r . 'float: none' -exclude-dir=.git
56.96s user 0.70s system 99% cpu 58.127 total
git grep 'float: none'
0.42s user 1.05s system 335% cpu 0.438 total
130x+ improvement :D
Just install grep 2.14, as recommended on this Hacker News thread.
I though I would have gotten better results :\
$ time grep -r icon-similarities
grep -r icon-similarities 0.11s user 0.75s system 12% cpu 6.646 total
$ time git grep icon-similarities
git grep icon-similarities 0.02s user 0.03s system 24% cpu 0.201 total