As a Haskell practitioner in industry - for several years we have been creating technical documentation sites for large enterprise customers, providing fast semantic search for gigabytes of content serving millions of users - I can attest that this simply isn't true. We have found that reasoning about performance in a pure functional language is not harder than for any other kind of mainstream language. Except in the sense that it is different, so not all of your skills and intuition from other languages necessarily apply.