The feature is built-in the Instant software and controllers, you will need a WebCC subscription to get and keep the classifications up-to-date.
You can imagine that maintaining reputation and web classification databases and keeping the cloud service available is something that takes quite some ongoing work. That is why a WebCC subscription is required.
There is no Anti-virus engine running on the AP/controller, the feature works through the WebCC cloud service on meta-data like destination, hostnames, etc, which is why it hardly affects performance.