I have used darcs a lot. But when our repo contained more then 1000 patches it becomes very slow. For example source file annotation took more than 2 minutes. I have tried optimizations, tagging etc. without bigger improvements. I still like darcs for its simplicity, cherry picking, great interactivity, etc.. However i need something a little bit faster. Another disadvantage is poor emacs support.

I suspect they're mixing theory with practice. The darcs theory is very good. But that doesn't mean you have to limit yourself to it. They could add memoization or git-style content-addressing as a backup to the patch theory.

I suspect that the majority of development and patch-rewriting happens in the most recent set of patches. There's probably a more than exponential drop-off in editing earlier patches.

Caching full repo / file information about earlier patches could speed darcs up to git-like levels.

Do you have a timeframe for these speed issues? darcs has been considerably sped up over the past years.

M-x package-install darcsum