The implementation has not progressed at all since 1999, when Harlequin folded. It doesn't do defunctorization, so probably can't compete with MLton on performance (but it does have an IDE including a REPL, unlike MLton). It's much closer to the Definition than SML/NJ (of course): once we have the compiler running again I'll be interested to use it on the MLton corner cases. And certainly the SML/NJ garbage collector was occasionally a subject of light-hearted mockery among MLWorkers. But as I say, I don't expect x86 performance to be all that great.