The problem with Prolog is that to get reasonably performant code at all you have to understand the underlying unification algorithm and manipulate it using cut

The problem with Prolog is that to get reasonably performant code at all you have to understand the underlying unification algorithm and manipulate it using cuts to limit backtracking. This means that you have to break the core abstractions of the language in order to do anything, which defeats the purpose of logic programming. Functional programming in Haskell makes it fairly easy to write fast code, though Haskell in particular is tricky to reason about space-wise (due to laziness).