Ocaml has way more paid development time than SML. MLton guys can't focus on everything, so they've decided final performance is more important than compile time. SML was designed with few ambiguities to the syntax (simpler for students to implement), so theoretical compile times could be very low indeed. If they had Google money behind them, compile times would definitely be better

The other thing is whole-program optimization vs method-at-a-time. A lot of SML devs use New Jersey for fast compiles when developing and MLton when deploying. You can get rid of a ton of stuff in functional languages once you do a whole program compile, but that will always take more time. MLton could really use a second, faster compiler, but I'm just appreciative of all their effort to make the current compiler output fast code.