Why is this Haskell IO Code so slow?

Question

I'm just starting to try to figure out IO and lazy IO in Haskell, and this example I wrote is really slow. It just reads a file of 10M integers and computes the maximum.

import System.IO

maxNum :: [String] -> Int
maxNum = maximum . map getInt
    where getInt x = read x :: Int

main = do
    inh <- openFile "randints.txt" ReadMode
    contents <- hGetContents inh
    let myMax = maxNum $ lines contents
    putStrLn $ show myMax

The simple Python version I wrote runs about 6 times faster:

inh = open('randints.txt', 'r')

my_max = max(int(line) for line in inh)

print my_max

hGetContents is supposed to be lazy, so I don't think it's reading the whole file into memory first. The profiler shows that 93% of the time is spent in maxNum.getInt, but I'm not experienced enough (or at all) profiling Haskell to figure out whether that's a red herring because of lazy evaluation.

Did I do something wrong that broke the laziness somehow? Or do regular Strings in Haskell just perform that poorly?