Administrator Mode - Implications on Security, Performance or what?

Well, I think Edge is aiming to be a security-oriented browser (unlike Chrome), and that's why it's ideal for Edge to remind people to follow best practices with regards to security. Perhaps they're mentioning performance rather than security in order to draw people's attention to it without alarming them too much? Not sure why it would be a performance issue rather than a security one.

I think there should be at least one prompt reminding people that it's a risky practice, but if you're getting recurring prompts throughout the browsing session that could be an issue.