[madgar]

JS is single-threaded which makes an enormous difference to actually squeezing performance out of your JIT.

Just building a JIT for a language generally isn't the hard part. Building a JIT that is substantially faster than a bytecode-compiled implementation of the language is what's hard, and how hard that is depends intimately on the semantics of the source language. When I say intimately, I mean every single detail of the language's semantics matter.