# Performance (/docs/guides/performance)



## What runs [#what-runs]

* **One** `MutationObserver` that watches for shadcn parts appearing and changing state
* **Two** listeners in the capture phase, `pointerdown` and `keydown`, that remember the last press
* Animations of `transform`, `opacity` and `filter` only, which the browser runs off the main thread where it can
* Ghosts, the copies of leaving elements, which are removed as soon as their animation ends or is cancelled

Nothing runs while nothing changes.

## Size [#size]

The layer is about 7 KB gzipped once your bundler minifies it: `cuts.tsx`, its stylesheet and the engine. Each helper adds about 1.5–2.5 KB.

## Lists [#lists]

ListCut measures its items once per render, in a layout effect. Only items in or near the viewport animate, so a list of thousands stays fast. Key items by a stable id, and avoid re-rendering the whole list on every keystroke elsewhere on the page.

## Slow devices [#slow-devices]

Blur is the most expensive part of a rack focus. If you target low-end phones, turn it off:

```css
:root {
  --kf-blur: 0px;
}
```