> Julia uses a method JIT, which are quick to implement, but which makes latency gets bad. Meaning if you load a whole new package, running the first function can cause some delay. With a JavaScript style tracer JIT added into the mix one could have probably managed to have both high performance and low latency.

This is actually the other way around -- Method JITs are harder to implement and all the major JavaScript JITs are method JITs, not tracing JITs. There are many reasons that contribute to the latency but I don't see what makes method JITs inherently slower than tracing JITs.