Add page about performance to docs

packages/docs/src/content/docs/guides/performance.md

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+---
+title: Performance
+---
+
+This page describes a few topics around Knip's performance, and how you might
+improve it.
+
+Knip does not want to tell you how to structure files or how to write your code,
+but it might still be good to understand inefficient patterns for Knip.
+
+## Star Imports and Barrel Files
+
+Knip builds up a simplified graph of imports and exports and can quickly match
+them against each other to find unused exports. However, there might not be a
+literal match for exports that are imported using the `import *` syntax. In this
+case, Knip will ask the TypeScript compiler to find references, which is a lot
+more work. More levels of re-exports and star imports are more expensive.
+
+Barrel files with re-exports look like this:
+
+```ts
+export * from './model';
+export * from './util';
+```
+
+Example of a star import:
+
+```ts
+import * as MyNamespace from './helpers';
+```
+
+Use the [`--performance` flag](../reference/cli.md#--performance) to see how
+often `findReferences` is used and how much time is spent there.
+
+This article explains the issue in more detail: [Speeding up the JavaScript
+ecosystem - The barrel file debacle][1]. The conclusion: "Get rid of all barrel
+files".
+
+## Workspace Sharing
+
+Knip "bundles" files from separate workspaces into a single TypeScript program
+if the configuration in `tsconfig.json` allows. This saves memory and time. The
+relevant compiler options are `baseUrl` and `paths`:
+
+- If the `compilerOptions.baseUrl` is not set explicitly
+- If there are no conflicting keys in `compilerOptions.paths`
+
+With the `--debug` flag you can see how many programs Knip uses. Look for
+messages like this:
+
+```sh
+...
+[*] Installed 2 principals for 29 workspaces
+...
+[*] Analyzing used resolved files [P1/1] (123)
+...
+[*] Analyzing used resolved files [P1/2] (8)
+...
+[*] Analyzing used resolved files [P2/1] (41)
+...
+```
+
+The first number in `P1/1` is the number of the program, the second number
+indicates additional entry files were found in and added to the same program
+during the first round.
+
+## GitIgnore
+
+Knip looks up `.gitignore` files and uses them to filter out matching entry and
+project files. This increases correctness. Your project may have multiple
+`.gitignore` files across all folders. It slows down finding files using glob
+patterns and in some cases significantly.
+
+You might want see if it's possible to disable that with `--no-gitignore` and
+enjoy a performance boost.
+
+To help determine whether this trade-off might be worth it for you, first check
+the difference in unused files:
+
+```sh
+diff <(knip --no-gitignore --include files) <(knip --include files)
+```
+
+And to measure the difference of this flag in seconds:
+
+```sh
+SECONDS=0; knip > /dev/null; t1=$SECONDS; SECONDS=0; knip --no-gitignore > /dev/null; t2=$SECONDS; echo "Difference: $((t1 - t2)) seconds"
+```
+
+Analysis on a sample large project went down from 33 to 9 seconds (that's >70%
+faster).
+
+## A Last Resort
+
+In case Knip is unbearable slow (or even crashes), you could resort to [lint
+individual workspaces][2].
+
+[1]: https://marvinh.dev/blog/speeding-up-javascript-ecosystem-part-7/
+[2]: ../features/monorepos-and-workspaces.md#lint-a-single-workspace