Technically app doesn't affect your page performance negatively because a technique we use to load your dedicated FAQ page.
By default when app is installed, we don't update, add or remove anything from your theme files so there is nothing to remove upon un-installation.
If you were on Professional plan, app has a feature where you can embed FAQs on different parts of your store. So when you un-install an app, you can remove embed code you've added yourself. But again technically even if you leave those embed snippets as it, it wouldn't affect page performance in any way.
If you have any questions around this, feel free to reach out to support.