devise benchmark that solely shows the improvement due to multi-threading

[willp]

This won't be a "fair" benchmark, but like, are any of them?

Basically, what's the benefit of using multi-threading at_all ? That's what this benchmark should show. Not a comparison against other multi-isolate libraries (haven't seen any benchamrks there), but instead benchmark the difference between an async-only single-threaded app, vs. on that uses BgThread Isolates.