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.
2. Split up long tasks.
3. 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?

[patrickhulce]

We think @paulirish would be a great candidate to tackle this new audit, tentatively called “all of Lighthouse” 😉

[midzer]

Maybe we could check how much % of JS is actually run on audited page and give a hint that X % is parsed and compiled without effect. Splitting up large bundles could be a major benefit for non-SPA apps on low end devices.

https://github.com/GoogleChrome/puppeteer/blob/master/docs/api.md#class-coverage should provide necessary information for this approach.

edit: Code splitting is good practice for SPA, too. For example webpack recommends https://webpack.js.org/guides/code-splitting/