Brainstorm: opportunities to improve main thread

Right now we do have diagnostics which summarize some main thread activity (`main thread breakdown` and `bootup time`), but we don't have any targeted opportunities regarding interactivity.

It'd be nice to help the users figure out how to improve the health of their main thread.

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Few general approaches:

1. Do less work
- Don't do the work at all (Find needless stuff and nuke it)
- Do the same work faster. (Profile it and speed it up.)
- Do the work off the main thread.
1. Split up long tasks.
1. Do the work in downtime
* Defer work till later, after TTI
* Do work earlier, so TTI doesn't get pushed out

Maybe we can develop some heuristics to create specific suggestions?