* Separate compilation vs whole-program compilation. OCaml uses separate compilation and therefore has a very constrained heap object model which makes it possible to get polymorphism across separately compiled modules. In contrast, MaPLe uses whole-program compilation and therefore is able to monomorphize and optimize much more aggressively. Whole-program compilation can be slow for large projects.
* The multicore OCaml effort was driven by backwards compatibility, especially in terms of performance -- they wanted to ensure that the performance of existing sequential OCaml code would be completely unaffected by the new run-time system. In contrast, MaPLe focuses on efficiency and scalability for parallel code.
* Multicore OCaml will let you implement your own scheduler, as a library, on top of coarse-grained threads. In contrast, MaPLe comes with a built-in scheduler, and it's not easy to change it.
We did a comparison with multicore OCaml in https://dl.acm.org/doi/10.1145/3591284, and found that MaPLe can be significantly faster, but that comes with all of the tradeoffs above. And, it's a cross-language comparison, so take it with a grain of salt. Our comparison in particular emphasized similar source code, but typically, fast code in OCaml just looks different from fast code in MaPLe. For example, in OCaml, you often need to manually unbox certain data structures to get better memory efficiency (but MaPLe will often do this for you, automatically).
One of the people who helped optimise the multi core implementation for OCaml said it was the way to go, but that was in 2020. Don’t know where things are now. https://news.ycombinator.com/item?id=23776609
