indeed, the PHP, Haskell and C# version have undue (since this kata is about how to define mutually recursive functions, not how to optimize the sequence generation) performance tests

there is an open issue regarding this

Hi. As you can see below, this issue has been opened 3 years ago by Unamed, first about Haskell (disproportionate requirements with original kata, and relatively to a 6kyu). Other users then reported that the same problem existed in C# and PHP. I close the issue since it's useless to have duplicate ones. Hopefully one day someone makes new versions more reasonnable in those languages.