LCP is promising for advanced needs.
However, I think it's missing the point by focusing on one element. Several adjacent elements painted almost simultaneously are not considered to be the same visual element, whereas for a user, they are (first Gestalt law, regarding proximity).
In addition, the metric is complex to implement as, for example, every image needs to have a Timing-Allow-Origin HTTP header. The server configuration required for that will only be achieved by people who are already involved in Web Performance.
The Start Render is much easier to evaluate and can be collected on any public page right now without any implementation costs. Much easier to compare your web performance to others.
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