But why would the system webview be less resource hungry? If it runs in an own process instance, the only memory you safe is (maybe) the executable code (i.e. maybe 100MB max).

Or you would need some common runtime which is shared across the OS, and somehow use IPC to it. Then you can potentially save some more memory. But I don't think this exists. And I'm also not sure how much you really would save.

The advantage of using the system webview is that you can update this component, in case there are any vulnerabilities or so.