It's amazingly difficult to find the right metric when you are still at the MVP level.

However, once the system has proven itself, it's only a matter of optimization.

I think it's one (amongst many) reason why so many startups fail at the MVP stage. Looking for metrics at that stage is just speculation; optimizing for them is a complete waste of time. The focus of the MVP is to build a proven system without getting lost (minimal waste of time/effort/resources).