-O/-O2 causes program to run too slow

I have a program I wrote to submit for the Car Game problem on Kattis: https://open.kattis.com/problems/cargame but it runs over the 5-second time-limit I downloaded the test data and found that on GHC 7.8.3, if I switch from -O2 to -O0, it runs three times faster (almost certainly fast enough for Kattis to accept). Can someone tell me what's going on? Is this a bug?