Performance

React Docs: Optimizing Performance
Debugging React performance with React 16 and Chrome Devtools
Chrome DevTools Docs: Analyze Runtime Performance

Server Request Profiling

The profiler is temporarily disabled due to non-compatibility with the current Node version.

We've included v8-profiler-next which allows you to generate CPU profiles (including flamegraphs) for requests to the Calypso NodeJS server. This is helpful for finding functions which impact performance the most on a given route.

To use the profiler:

Run USE_SERVER_PROFILER=true yarn start.
Visit a url like calypso.localhost:3000/themes
After waiting a few extra seconds, a profile file is saved to "./profiles/$route/$route-$time.cpuprofile"
This profile can be viewed with VS Code's built-in profile viewer by clicking on the profile in VS Code's file explorer. If you click the flame icon in the upper-right corner, VS Code will prompt to install a flamegraph viewer extension as well.

Some notes:

The behavior of the dev server can differ from production, and having the profiler enabled can reduce performance. While profiles should not be treated as a source of truth for absolute production performance, they are still useful for seeing relative performance. (E.g. to find a function which takes relatively more time than other functions.)
Any slash in the URL ("/") is changed to "_" in the filename. So when you access the base route ("/"), the profile will be saved to "./profiles/_/_-$datetime.cpuprofile"
Only one profile can be generated at a time. If you visit another route at the same time a profile is being generated for a different route, a new profile is not created. However, since the CPU is a shared resource, the impact of visiting the second route at the same time will still be visible in the first route's profile.
Requests to various static and dev resources are not profiled.

Bundle Analysis

Why is X bundled?

If you want to know why a certain module is bundled you can use whybundled to find out. See the following for an example on usage:

yarn run preanalyze-bundles
yarn run whybundled -- [module]

npn run whybundled -- is-my-json-valid

This should give you an overview on where this module got bundled and which file are requiring it:

MODULE  is-my-json-valid
├─ imported: 13 times
├─ deps count: 5
├─ size: 19 KiB [for all included files]
├─ type: [direct]
├─ chunks: vendors~build
├─ locations:
│  └─ ./node_modules/is-my-json-valid/
│
├─ files:
│  ├─ ./node_modules/is-my-json-valid/formats.js
│  └─ ./node_modules/is-my-json-valid/index.js
│
└─ reasons:
  ├─ ./client/extensions/woocommerce/index.js + 439 modules  7:0-41  [harmony side effect evaluation]
  ├─ [...]