Maybe a before and after, but one that focus on expressiveness, what you were not able to say/express in OCaml before, that is now* possible
not just speed or performance
*s/not/now
https://github.com/ocaml-multicore/effects-examples has links to tutorials and examples for how effects can be used.
There's also some slides from KC's talk on effect handlers https://kcsrk.info/slides/handlers_edinburgh.pdf and materials from the CUFP 17 tutorial: https://github.com/ocamllabs/ocaml-effects-tutorial
https://gopiandcode.uk/logs/log-bye-bye-monads-algebraic-eff... this is also a great introduction