As I understand it, that delegate method is called whenever an extension rule matches something on the page.

That would be amazingly slow to call an out of process function on each match when the page renders. Extensions run as a separate process. I would be amazed if Apple would allow something that inefficient as careful as it is about battery and memory efficiency.

I mean, the Safari application is already doing XPC with the web content process constantly anyways. And perhaps they can batch these messages?

Why would Safari be doing any communication with the content blockers? The content blockers are submitting JSON one time to iOS Safari and aren’t running at all.

The Safari App Extension is running in the background, which is presumably what the process is communicating with.

You’re talking about MacOS and not iOS? As far I know, the only time Safari or any other app on iOS calls an extension is when the user chooses to “share” with that extension

Yeah, I think this API is only available on macOS. On iOS this would require keeping the extension alive in the background, which Apple seems largely opposed to allowing.