Why does mercurial's “hg rebase” command take so much time for a relatively large repository?

Question

Even though the changeset itself is very small, when I do a "hg rebase" for a relatively large repository (say around 10GB) why does it so much time. What exactly goes on behind the scenes?

Answer 1

Two quotes from hg help rebase

If you specify neither "-s" nor "-b", rebase uses the parent of the working directory as the base.

+

If you specify base ("-b/--base"), rebase will select ancestors of base back to but not including the common ancestor with dest.

With used by you syntax and possibly DAG of repo selecting ancestors may be time-consuming task