Self-answering to some degree, it looks like WKWebView on macos is similar to safari but may be behind on version number (presumably updated only when the OS is updated) and thus slower.

You can also embed an actual safari view, but it doesn't let your control things, so it'd have to be more strictly a "dumb shell": https://developer.apple.com/news/?id=trjs0tcd

Of course, I'm not sure how either compares to electron, and I'm sure it depends on the app.