Hmm. Are you actually sure that the repos are in darcs-2 format and that you have
converted only a single branch? I somewhat doubt that this is the case, for
various reasons... Can you please give cat _darcs/format? If it says "hashed",
this is actually an old-style repository, just the content is hashed. If you have
obtained this with darcs convert (and done that correctly, without converting
related branches twice), then yes, you have likely found an exponential issue in
darcs 2 format (this I think is known to exist, but should be quite hard to
create conflicts in such a way to trigger it).

Thank you.