I'm not sure. From the description, it sounds like whatever rust does on one thread (compute + destructors) is split across multiple threads in Go (compute + gc), but it's not clear if the base computation workload is spread across multiple threads.

Thinking about the problem, I think that at the very least parsing could be parallelized. Assembling everything into one output might not be parallelizable. But I haven't looked at what happens in esbuild.

I do know that it's fast enough that when I switched from webpack I had to check that it actually did something, because it returned immediately.