It depends on what kind of performance problems you have.<p>Some algorithms are hard to express in pure, strict functional settings.  (There&#x27;s even an impossibility theorem for a few algorithms, but that theorem doesn&#x27;t apply to pure, lazy languages.)  That being said, the worst asymptotic slowdown is logarithmic---because you can always simulate random access memory.<p>On a more practical side, you have constant factors to worry about, and things like caches.  Look at eg Don Stewarts work for how to deal with these in a functional language.<p>(And, in practice, there&#x27;s no shame in throwing in the towel and either interfacing with other languages, or even switching to them.)