User engagement is a first moment of conversion. Will they stay or will they leave?
To optimize for the first user engagement, I look at the First Meaningful Paint as a performance and user experience metric (to be replaced by Largest Contentful Paint). Elements to check for, are product-title, price and image, as these are most meaningful towards visitors.
I did a comparison on a mobile device, fast 3G internet connection, measured from abroad (Dulles, United States).
Within this comparison, it is very clear how resources can be render-blocking. In the current situation, content will show on 6.4 seconds (being the First Contentful Paint and First Meaningful Paint metrics). In the optimized situation, we can see how browsers work. The First Contentful Paint is at 1.4 seconds. The First Meaningful Paint is at 1.7 seconds. This is an improvement of 73%.