The current measures
Largest Contentful Paint for loading, Interaction to Next Paint for responsiveness, and Cumulative Layout Shift for visual stability.
The one most sites fail
Layout shift, and usually for a mundane reason — images without dimensions, ads injected above content, or fonts swapping late.
Measure with field data
Lab tools are useful for diagnosis but real-user measurement is what search engines use. A site fast on your laptop can be slow on a mid-range Android phone.