ndeloof/mercurial-plugin — comment on commit d5f544101dd1083c4ef11a28dd88941bb7d887c5
File:  line 0
By rhass:

In large repositories (hundreds of thousands of files) we have found passing the --rev option is a very expensive operation as mercurial has to calculate the deltas from the upstream repository. This process takes a significant amount of time and resources versus a full clone and updating the working copy to a specific branch, cset, or tag.

I realize this information exceeds the scope of this patch; however it is important to be aware of this tax going forward.