Using pure functional languages can have a lot of benefits over using impure imperatives, but low level systems languages will generally allow you to achieve much greater performance, especially when they are imperative because it allows you to specify the exact steps in how the cpu should compute the result.
If there is ever list of tools where high performance is an absolute must then I would put VCS at the top of that list, and git achieves this very well. However performance is not git's only advantage over many other other types of version control systems anyway.
The git team is handling the unsafe c code very well, and I never worry about my type system (or any other features of the language it is written in), so why is it that there is a lot of haskell developers that must use darcs when they will only be using the finished product?