This version is about 2x as fast as OP's on my system (look at h2.hs): https://gist.github.com/gregorycollins/9103248 

$  time ./dist/build/h1/h1 50000000 +RTS -A4M > /dev/null

./dist/build/h1/h1 50000000 +RTS -A4M > /dev/null  23.88s user 0.06s system 99% cpu 24.003 total

$  time ./dist/build/h2/h2 50000000 +RTS -A4M > /dev/null

./dist/build/h2/h2 50000000 +RTS -A4M > /dev/null  13.57s user 0.09s system 99% cpu 13.706 total

$  time ./dist/build/h3/h3 50000000 +RTS -A4M > /dev/null

./dist/build/h3/h3 50000000 +RTS -A4M > /dev/null  24.24s user 0.18s system 99% cpu 24.490 total

The "system" number here is especially telling: this program is spending almost all of its time in syscalls.