Do more benchmarking on ScalaJS tracing

[djspiewak]

It's important to convince ourselves that we haven't somehow entirely destroyed performance on scalajs. The benchmark results are really inconsistent and it would be immensely helpful if someone who understands V8 and some of ScalaJS were able to help out on this a bit, since it's difficult to explain the numbers we're seeing in #2207.

Running hey benchmarks on http4s-server and even perhaps the Fs2 benchmarks on ScalaJS would probably be the best way to start to get some confidence over how this change manifests in realistic workloads.

[djspiewak]

Requires #2684

[armanbilge]

V8 performance notes, but might be a bit dated: https://github.com/thlorenz/v8-perf