Seems like on the average it does offer a boost in performance.
And there is some aditional work providing javascript parsers for rust (which you could build tools like babel on top of): https://github.com/dherman/esprit
I think part of BuckleScript's speed may come from the source language, OCaml, being easier to parse. There may also be fewer passes necessary to transform OCaml to JavaScript.
BuckleScript using Reason (with a similar syntax than JS, slower parser than vanilla OCaml) is still one of the fastest around. Mostly a case of very careful and dedicated engineering here (I work on Reason with the author of BuckleScript). Additionally, we try to work smart and delegate most of the build process to Ninja, which itself is one of the fastests around.
But I believe the topic here was about runtime performance of using a language to compile JS, not about the build speed of working in that language itself. In which case you’ll still get some wins writing a JS toolchain in BuckleScript (compiled to JS), just from the JiT-friendliness of the BuckleScript JS output.
But realistically, you’d be compiling to native OCaml through the same codebase. We did see a 10-25x perf jump from converting a part of a Babel pipeline to native OCaml. I mean, these languages are basically designed over decades with AST manipulation in mind, so that’s not surprising.