Unfortunately darcs suffered from the patch-of-doom problem, where if you were unlucky an exponentiation merge would kill all performance of your repository.

The darcs FAQ from 2012 reports it as still unfixed:

http://darcs.net/FAQ/Performance#is-the-exponential-merge-pr...

(Yes this was real, and yes we hit it. I liked the patch-centric darcs approach, but that was enough to make us migration to mercurial, then later git.)