I have to work with an hg repository that has millions of lines of code and hundreds of thousands of changesets. As you can imagine, this really slows down mercurial and To
No you can't, but you can optimise your local clone. Have a look at my answer to https://stackoverflow.com/a/19294645/479199.
There has been some work on shallow clones, but it's still very much a work in progress (and there doesn't seem to have been much progress):
https://www.mercurial-scm.org/wiki/ShallowClone