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.
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.