> it claims to use zero CPU

There is no claim of "zero CPU". The claim is that the service worker wakes up only when necessary -- it is designed to be suspended by default from the ground up.

In Optimal and Complete modes, the content scripts will of course execute, without the service worker being unsuspended if no filtering occurs, but perform only the necessary work and bail out ASAP if not needed.

In Basic or "No filtering" modes, no content scripts are injected.

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Edit: Sorry, I do say "uBOL itself does not consume CPU/memory resources while content blocking is ongoing". When I say "itself" I am referring to the service worker as seen in Chromium's Task Manager. The service worker isn't required for examples when navigating to `example.com` or here at `news.ycombinator.com`. All top content blockers I have looked at do require their service worker to execute, even for merely just switching between tabs. Some even use tricks to prevent their service worker to be suspended at all.

Thank you for the explanation! That makes sense now.

I'm still confused about what level of access is given to the extension and what using the extension means. Clicking on the extension asks for access to the current website, so I'm assuming that without giving access there or clicking "Always Allow on Every Website..." in the Safari settings, the extension does not have access to the web page contents.

Basic filtering claims to not require permission to read the web page data. But the extension is still used and does content filtering right?

Maybe this is more of a comment on Safaris weird terminology in the permission settings.