[bob_roboto]

Immutable persistent data structures used in FP tend to perform slightly worse than their "conventional" counterparts. So if you can manage to run your non-FP program in parallel with a shared mutable data structure and no synchronisation you should see better performance. In practice, however, you'll almost certainly need synchronisation on mutable shared state and it might very well be slower. So neither approach is per se faster than the other.

[barrkel]

There are ways to encode imperative mutable updates to data structures in a functional style with tail calls and sufficient compiler smartness. I don't think you really come out ahead if you go down that route though.