Does Feasible slow my site down?

Not by any amount you could measure.

Four reasons, all structural rather than promises.

The tag is deferred. It never blocks parsing or rendering, wherever you put it. Nothing on your page waits for us.

It’s one request of 3,377 bytes gzipped, cached for an hour with an ETag, so most loads are a 304 with no body. There’s no second file, no config fetch, no vendor chain pulling in three more domains.

There’s no polling timer. Time on page and scroll depth accrue from browser events, not from something waking up every second to check. A tab sitting open costs nothing.

Sends are fire-and-forget. Events go out with fetch and keepalive, so nothing waits on a response and a slow network never holds up navigation.

Two things it does that help the numbers:

It defers everything until the page is visible, so a prerendered page or a tab opened in the background doesn’t do tracking work while you’re trying to hit a Largest Contentful Paint target. And it never calls preventDefault on an outbound link or a form submit - some analytics scripts delay the navigation to squeeze a request in, which is a real, felt slowdown. This one uses a hold-open fallback instead and gets out of the way.

If you want to put a number on it, load your site with the tag and without it in a lab tool and look at the diff. It’ll be inside the noise.

The one thing that can cost you is a tag manager. A container script is usually several times the size of this one, and it’s loading a tag you could have pasted directly. Paste the tag.