Along with the graphs from the PR in the sibling comment, there's also the extensive benchmarking from the ICFP2020 paper: https://arxiv.org/pdf/2004.11663.pdf

Work on this is on-going via the sandmark benchmarking suite: https://github.com/ocaml-bench/sandmark

In short the expectation should be that single-threaded code performs roughly the same (single digit percentage changes) as on the sequential runtime.

Parallel code on multicore can see close to linear speedups on 64 cores, though it depends significantly on your workload. If you're interested in parallelising existing OCaml code, I gave an example-driven OCaml workshop talk in 2020: https://www.youtube.com/watch?v=Z7YZR1q8wzI

Yes, have a look at [0] and maybe also the one in [1]

[0]: https://github.com/ocaml-bench/sandmark [1]: https://github.com/ocaml-multicore/retro-httpaf-bench/pull/1...