Here are the results of profiling a self compile. mlprof -s -t 1 mlton mlmon.out 522.19 seconds of CPU time forward (C @ 0x85BC034) 15.42% <unknown> 13.72% output_8 9.47% GC_foreachPointerInObject (C) 6.62% toLiveness_1 3.97% eq_0 3.81% toString_45 3.43% GC_foreachPointerInRange (C) 1.94% eq_2 1.82% concat_0 1.66% post_1 1.31% chooseRegister_1 1.27% utilized_0 1.19% x_17447 1.16%