You might be thinking JavaScript runs on a single threaded event loop? That's correct, however you can run N event loops in parallel (isolates in v8 lingo). `deno serve` even has a `--parallel` flag (https://docs.deno.com/runtime/reference/cli/serve/).

It would have certainly been simpler to just plot the overall runtime (width of the graph), I did think that it was quite interesting that PB's goja integration takes a while before utilizing the entire machine.