Why is ScreenCaptureKit a bad choice for performance?

Because you can't control what the content server is doing. SCK doesn't care if you only need a small section of a window: it performs multiple full window memory copies that aren't a problem for normal screen recorders... but for a utility like mine, the user needs to see the updated content in milliseconds.

Also, as I mentioned above, when using SCK, the user cannot minimize or maximize any "watched" window, which is, in most cases, a deal-breaker.

My solution runs at under 2% cpu utilization because I don't have to first receive the full window content. SCK was not designed for this use case at all.