Hi, I know there's been quite some performance/optimization post lately, so I hope there still room for one more. While solving a ProjectEuler problem (27), I saw a performance issue I cannot explain. I narrowed it down to the following code (never mind that 'primes' is just [1..], the problem is the same or worse with real primes): primes :: [Int] primes = [1..] isPrime :: Int -> Bool isPrime x = isPrime' x primes where isPrime' x (p:ps) | x == p = True | x > p = isPrime' x ps | otherwise = False main = print $ length (filter (== True) (map isPrime [1..5000])) $ time ./experiment1 5000 real 0m4.037s user 0m3.378s sys 0m0.060s All good, but if I change isPrime to the simpeler isPrime x = elem x (takeWhile (<= x) primes) it takes twice as long: time ./experiment2 5000 real 0m7.837s user 0m6.532s sys 0m0.141s With real primes, it even takes 10 times as long. I tried looking at the output of ghc -ddump-simpl, as suggested in a previous post, but it's a bit over my head (newby here...). Any suggestions? Thanks a lot, Jeroen