I use a way provided by Mozilla (documented here: https://developer.mozilla.org/en-US/docs/Developer_Guide/Source_Code/Mercurial/Bundles) to speed up the cloning of the repo dramatically.

We download a daily generated ~500mb hg bundle instead of thousands of files one by one with a total size of ~2.3 gb (as hg clone would do) and then update only the most recent changes.

My idea was to do something like: 
```ruby
head do
  url 'http://ftp.mozilla.org/pub/mozilla.org/firefox/bundles/mozilla-central.hg', :using => HgBundleDownloadStrategy
  repo 'https://hg.mozilla.org/mozilla-central/'
end
```

and replace the hardcoded repo url in line 21 with the value of the repo parameter.