Figure out a easy way to profile time-to-interactive

We currently don't have a simple way to profile time-to-interactive.

This will involve figuring out what the actual thing we want to measure is and how to measure it, as well as probably running against a local server, to make sure that we can control network latency.

Ideally the end result of this issue would be a doc in `docs/` and possibly a tool or patch to make measuring this simple.