Can we improve the startup time of our simulations?

[samreid]

Inspired by #338 (comment) and https://github.com/phetsims/phet-io/issues/1733, I'm wondering how much faster we could get our simulations to launch without sacrificing features.

Is the goal faster startup for built sims? Faster startup for unbuilt? Faster startup for running generate-phet-io-api? Something else? This seems to be an epic-level issue, but not necessarily for the near future. Tagging for potential discussion as part of #678