When you have a Webview hosting an iframe element, it's possible to monitor what's going on in the iframe using the MSWebViewFrame* events, which are documented with the x-ms-webview element. These are MSWebViewFrameNavigationStarting, MSWebViewFrameContentLoading, MSWebViewFrameDOMContentLoaded, and MSWebViewFrameNavigationComplete.
In the event you have multiple iframe elements in a webview, the way to distinguish between frames is with the e.uri property, where e is the event args object for the event in question. This property is presently undocumented, but should be–this comes directly from the engineers who built the webview, so we can trust that it's there!
Generally speaking, the content of a Webview is generally a black box from the host app's perspective, and for this reason you can't get into the Webview's DOM, you can't get element references (e.g. to an iframe), and so on. There'd be serious overhead to go that route.
However, there is the possibility that long-running scripts within whatever web page you're hosting are taking so long that your app is considered to be unresponsive. Those are messages you've seen in the browser as well, and if the app gets blocked on the same kind of script, it's possible the Windows will take the app down. This is a known issue that might be addressed in future releases of the operating system. At present, you can consider watching the Webview/iframe NavigationStarting and setting up your own timeout so you can catch an unresponsive site before Windows takes down the app.