[dev docs] Port performance docs to new docs system.

dev_docs/key_concepts/performance.mdx

@@ -0,0 +1,106 @@
+---
+id: kibDevPerformance
+slug: /kibana-dev-docs/performance
+title: Performance
+summary: Performance tips for Kibana development.
+date: 2021-09-02
+tags: ['kibana', 'onboarding', 'dev', 'performance']
+---
+
+## Keep Kibana fast
+
+*tl;dr*: Load as much code lazily as possible. Everyone loves snappy
+applications with a responsive UI and hates spinners. Users deserve the
+best experience whether they run Kibana locally or
+in the cloud, regardless of their hardware and environment.
+
+There are 2 main aspects of the perceived speed of an application: loading time
+and responsiveness to user actions. Kibana loads and bootstraps *all*
+the plugins whenever a user lands on any page. It means that
+every new application affects the overall _loading performance_, as plugin code is
+loaded _eagerly_ to initialize the plugin and provide plugin API to dependent
+plugins.
+
+However, it’s usually not necessary that the whole plugin code should be loaded
+and initialized at once. The plugin could keep on loading code covering API functionality
+on Kibana bootstrap, but load UI related code lazily on-demand, when an
+application page or management section is mounted.
+Always prefer to import UI root components lazily when possible (such as in `mount`
+handlers). Even if their size may seem negligible, they are likely using
+some heavy-weight libraries that will also be removed from the initial
+plugin bundle, therefore, reducing its size by a significant amount.
+
+```ts
+import type { Plugin, CoreSetup, AppMountParameters } from 'kibana/public';
+export class MyPlugin implements Plugin<MyPluginSetup> {
+  setup(core: CoreSetup, plugins: SetupDeps) {
+    core.application.register({
+      id: 'app',
+      title: 'My app',
+      async mount(params: AppMountParameters) {
+        const { mountApp } = await import('./app/mount_app');
+        return mountApp(await core.getStartServices(), params);
+      },
+    });
+    plugins.management.sections.section.kibana.registerApp({
+      id: 'app',
+      title: 'My app',
+      order: 1,
+      async mount(params) {
+        const { mountManagementSection } = await import('./app/mount_management_section');
+        return mountManagementSection(coreSetup, params);
+      },
+    });
+    return {
+      doSomething() {},
+    };
+  }
+}
+```
+
+### Understanding plugin bundle size
+
+Kibana Platform plugins are pre-built with `@kbn/optimizer` 
+and distributed as package artifacts. This means that it is no
+longer necessary for us to include the `optimizer` in the 
+distributable version of Kibana Every plugin artifact contains all
+plugin dependencies required to run the plugin, except some
+stateful dependencies shared across plugin bundles via 
+`@kbn/ui-shared-deps`. This means that plugin artifacts _tend to
+be larger_ than they were in the legacy platform. To understand the
+current size of your plugin artifact, run `@kbn/optimizer` with:
+
+```bash
+node scripts/build_kibana_platform_plugins.js --dist --profile --focus=my_plugin
+```
+
+and check the output in the `target` sub-folder of your plugin folder:
+
+```bash
+ls -lh plugins/my_plugin/target/public/
+# output
+# an async chunk loaded on demand
+... 262K 0.plugin.js
+# eagerly loaded chunk
+... 50K  my_plugin.plugin.js
+```
+
+You might see at least one js bundle - `my_plugin.plugin.js`. This is
+the _only_ artifact loaded by Kibana during bootstrap in the
+browser. The rule of thumb is to keep its size as small as possible.
+Other lazily loaded parts of your plugin will be present in the same folder as
+separate chunks under `{number}.myplugin.js` names. If you want to
+investigate what your plugin bundle consists of, you need to run
+`@kbn/optimizer` with `--profile` flag to generate a
+[webpack stats file](https://webpack.js.org/api/stats/).
+
+```bash
+node scripts/build_kibana_platform_plugins.js --dist --no-examples --profile
+```
+
+Many OSS tools allow you to analyze the generated stats file:
+
+* [An official tool](https://webpack.github.io/analyse/#modules) from
+Webpack authors
+* [webpack-visualizer](https://chrisbateman.github.io/webpack-visualizer/)
+* [webpack-bundle-analyzer](https://github.com/webpack-contrib/webpack-bundle-analyzer)