Webperf docs (#879)

* Webperf docs

* Apply suggestions from code review

Co-authored-by: Kasia Zadurska <katarzyna.zadurska@allegro.pl>

Co-authored-by: Kasia Zadurska <katarzyna.zadurska@allegro.pl>

docs/index.md

@@ -32,4 +32,8 @@ How to customize Turnilo, plugins, query and request decorators.
 
 ## [OAuth integration](oauth.md)
 
-How to integrate Turnilo with OAuth provider
+How to integrate Turnilo with OAuth provider.
+
+## [Web performance](webperf.md)
+
+How to take care of a web performance.

docs/webperf.md

@@ -0,0 +1,49 @@
+# Web performance
+
+* TOC
+  {:toc}
+
+## Overview
+
+An important aspect of Turnilo is its web performance.
+Upon every change to the codebase we should track how performance changes.
+
+## JS bundle size
+
+Bundle size can easily inflate, particularly when an app has plenty of dependencies, just like Turnilo.
+With every change we should keep track of the bundle size, especially when we add a new dependency.
+
+Before adding a new dependency, please consider a smaller alternative in terms of bundle size.
+Tools like [Bundlephobia](https://bundlephobia.com/) will help to recon cost of library and find alternatives.
+
+### Size-limit
+
+[Size-limit Github Action](https://github.com/marketplace/actions/size-limit-action) will help to stay with assets size in the budget.
+On each pull request this action will post a comment with current bundle size and its delta.
+
+**Each time budgets are exceeded CI will fail.**
+
+You can adjust budgets in `size-limit` section of `package.json`.
+
+### Bundle analysis by Statoscope
+
+On every build a report about Webpack's bundle is made by [Statoscope](https://statoscope.tech/).
+You can find these under `build/report-*.html`.
+Among others it offers detailed tree-map of the client bundle.
+For example it helps to figure out which dependencies are the heaviest.
+
+### Transpiling dependencies
+
+Usually pre-transpiled dependencies are bad for bundle size, since they can include utilities like Babel's helpers or TS' `tslib`. It's a problem because stuff like this is unnecessary for modern browsers and hence it should not be included in the bundle.
+If possible import dependencies from an untraspiled source.
+Any dependency that has to be transpiled should be [listed within Webpack configuration](../config/webpack.common.js#28).
+
+### Manual (dead) code elimination
+
+Sometimes we can't rely on libraries' authors or on Webpack in terms of tree-shaking aka dead-code-elimantion and we have to take matters into our own hands.
+
+Webpack by [`IgnorePlugin`](https://webpack.js.org/plugins/ignore-plugin/) allows to drop selected modules and this [how Moment's locales are not included in the final bundle](../config/webpack.common.js#45).
+
+## Lighthouse
+
+On each pull request [Lighthouse-CI](https://github.com/GoogleChrome/lighthouse-ci) action will post link to lighthouse report. It can help to measure current performance and notice potential performance issues.