Sorry for replying to myself, but I got suspicious about the 6ms runtime of the 64-bit C++ code on my machine. So I looked at the assembly and found this: .LCFI1: movabsq $499999999500000000, %rsi movl $_ZSt4cout, %edi pushq %r12 I'm no assembly guru, but that makes me think that there's no actual computation going on in the runtime for the 64-bit C++ program, whereas the 32-bit one is clearly doing work on my system, since it takes around 1 second. Not that I'd be sad if GHC could reduce that whole constant at compile time, but GCC isn't doing 1 billion adds in 6 (or even 60) milliseconds.