Multicore is coming along, you can read the latest news here: https://discuss.ocaml.org/t/multicore-ocaml-june-2021/8134

In terms of performance, there is this paper https://kcsrk.info/papers/system_effects_feb_18.pdf where on a single core async OCaml and effect OCaml are close to Go's net/http, and there is also this project https://github.com/ocaml-multicore/retro-httpaf-bench but I haven't see any results from it.

The last multicore prerequisite will land with 4.13 in September. My understanding is that the next release after that due in Q1 of 2022 will have multicore support.

It is coming along, however in the age of security and distributed computing, what most applications really want are multiple processes with OS IPC anyway.

I was under the impression that multicore would help for the dev tooling world, so things like Pyre, Hack, etc.

Minor improvements, they can already make use of something like Lwp today.